In this article we will discuss about the dissection of cuttlefish. Also learn about:- 1. Dissection of Alimentary System 2. Dissection of Nervous System 3. Dissection of Reproductive System.
Dissection of sepia (Fig.12.1) is best done in specimens properly preserved in formaldehyde or spirit. Unlike other invertebrates, in sepia, it is convenient to dissect alimentary and nervous systems from the dorsal side and the reproductive system from the ventral side.
Dissection:
The internal shell is embedded in the dorsal mantle and it is removed with the latter. To expose the pallial complex from the dorsal side hold the specimen on the palm of your left hand, keeping the dorsal surface upwards and the anterior end directed to you.
Pierce the mantle at the anterolateral corner with the pointed arm of a pair of stout scissors, at the junction of the trunk and neck and give a transverse incision along the neck, passing below the ridge formed by mantle.
Starting from the neck give two longitudinal incisions on the mantle, one on each lateral side, slightly dorsally, up to the aboral end and carefully remove the dorsal mantle by separating it from the underlying tissues. The pallial complex is exposed.
To expose the ventral pallial complex hold the specimen on the palm of your left hand, with the ventral surface upwards and the anterior end directed to you. Insert the pointed arm of a pair of stout scissors through the opening of the funnel, at the junction of the head and the trunk and give a longitudinal incision in the thick mantle up to the posterior (aboral) end of the trunk.
Starting from the anterior end of the incision, give two transverse incisions, along the neck, right and left, up to the lateral sides and the ventral mantle is free from the neck. Place the specimen on a dissecting tray and put sufficient water to submerge it.
Stretch the mantle flaps laterally and pin them down. If desired, the mantle of the ventral surface may be removed by giving longitudinal incisions along the lateral sides of the trunk. The organs in the mantle cavity are exposed.
Dissection of Alimentary System:
Starting from the neck give a medial incision in the head up to the bases of the arms. Pin down the flaps. The mouth and the fore gut are partly exposed. As the oesophagus runs through the aggregated nerve ganglia, enclosed in a cartilagenous cranium, it is necessary to remove the dorsal part of the cranium and the cerebral ganglia to fully expose it (Fig. 12.2).
Mouth:
A round aperture at the anterior end, surrounded by oral arms. The lip is circular and beset with many papillae. A pair of jaws are lodged in the lip.
Buccal mass:
Fairly large, pyriform and contains an odontophore.
Oesophagus:
A straight, narrow tube, runs posteriorly along the middle line, between the lobes of the digestive gland and joins the stomach, at about the middle of the trunk.
Salivary glands:
A pair of glands, located behind the cranial cartilage, one on either side of the oesophagus. Salivary ducts run inward and the two join in the middle. The common duct opens dorsally into the posterior part of the buccal cavity.
Stomach:
The stomach has two interconnected chambers, the opening between the two provided with a sphincter. The first part or gizzard is somewhat oval and muscular. The second part or caecum is round.
Digestive gland:
Commonly called liver. A large, brownish gland, divided into right and left halves, partly connected, and lie by the sides of the oesophagus. The ducts from two lobes open separately at the junction of the stomach and the intestine.
Pancreas:
Minute, cream coloured vesicles, clustered round the digestive ducts and their openings. The duct from the pancreas opens in the gizzard.
Intestine:
A round tube, arises from the caecum, bends sharply upon itself and runs anteriorly as the rectum.
Rectum:
It is of same diameter as the intestine, runs forward, parallel and ventral to the oesophagus and opens in the mantle cavity through anus, close to the internal opening of the funnel.
Dissection of Nervous System:
The nerve ganglia, paired cerebral, pleural, pedal and unpaired visceral ganglion are of large size, aggregated around the oesophagus and in contact with one another. They are enclosed in a cartilagenous cranium (Fig. 12.3).
The nervous system in sepia should be dissected from the dorsal side. Give a median longitudinal incision in the head, starting from neck to the bases of the arms. Pin down the flaps and the cranium is exposed. With the help of a fine scalpel and a pair of forceps remove the cranium in pieces and the nerve ganglia come in view.
Cerebral ganglia:
The two cerebral ganglia are fused to form a large round mass dorsal to the oesophagus. From each ganglion arise an optic nerve, a statocyst nerve and a cerebro- superior buccal connective.
Superior buccal ganglia:
A small pair, closely united and located dorsal to the oesophagus, close to the buccal mass. They are connected with the cerebral ganglia by cerebro-superior buccal connectives.
Inferior buccal ganglia:
A pair of small, closely united ganglia on the dorsal surface of the oesophagus, posterior to the superior buccal ganglia, with which they are joined by slender connectives. A loop of sympathetic nerves arising from the ganglia courses along the oesophagus to the stomach and each ends in the gastric ganglion.
Pleurovisceral ganglia:
The visceral ganglion is below and the pleural ganglia are on the lateral sides of the oesophagus. The pleurals and the visceral fuse to form a pleurovisceral ganglionic mass.
It gives off:
a. Two visceral nerves, which send branches to visceral organs and continue as branchial nerves to the gills.
b. Two pallial nerves to the mantle containing two stellate ganglia.
Pedal ganglia:
The two ganglia are fused and located below the oesophagus.
It sends:
a. Ten brachial nerves to the arms. The brachial nerves are connected by a ring commissure.
b. A pair of nerves to the funnel.
Dissection of Reproductive System:
The sexes are separate.
The reproductive system should be dissected from the ventral side. Following exposure of the pallial complex, carefully remove the structures ventral to the reproductive organs, viz. gills, kidneys, abdominal veins etc. The sex organs are exposed.
Male Reproductive System:
(Fig.12.4)
Testis:
A round, encapsulated structure at the aboral end of the body.
Vas deferens:
A long, narrow, greatly convoluted tube arises from the left side of the testis and ends in the seminal vesicle.
Seminal vesicle:
It is wide, elongated and further expands to form a large sac, the spermatophoral or Needham’s sac.
Penis:
The spermatophoral sac narrows down towards the extremity to form penis and pens in the left side of the mantle cavity at about the middle of the trunk through gonopore.
Prostate:
A somewhat ovoid, glandular body, appended to the seminal vesicle, prior to the formation of spermatophoral sac.
Female Reproductive System:
(Fig. 12.5)
Ovary:
An anteriorly truncate, encapsulated, oval mass lodged at the aboral end of the body.
Oviduct:
A short but wide tube, arises from the anterior end of the ovary towards the left side, runs straight forward and opens in the left side of the mantle cavity.
Nidamental glands:
A pair of somewhat oval glands, lie side by side, ventral to the ovary, towards its anterior end.