The abdomen is not simply confluent with the sexual body, but thrust by it completely forwards. The anus mostly lies in front of the middle of the body.
3314. The intestine separates for the first time distinctly into cephalic and sexual, or small and large, intestine.
3315. The vitelline canal is the cœcum. This is very distinct in the Sharks and Rays.
3316. In Birds therefore the vitelline canal is the cœcum also, and the two cœca, falsely so called, which are placed upon the side of the rectum cannot represent the cœcum coli, or else the Bird must have three of these blind appendages or sacs.
3317. The spleen here appears for the first time; while the pancreas divides into a number of cœcal appendages.
Thorax.
3318. The thorax of the Fish is reduced within very narrow limits; it is like the first thoracic formation, and is thus a Mussel's thorax. The branchiæ with their opercula are similarly formed to the branchial plates and shells of the Bivalve Mollusca. The thorax is therefore attached only externally to the body, and the Fish is to be viewed as a Mussel, from between whose shells a monstrous abdomen has grown out.
3319. But this Molluscan thorax is conjoined with animal systems, and has assumed their noble or elevated type of structure. Here therefore the osseous and sarcose system blend together, and the higher formation of the thorax emerges into view.
3320. For the first time an advance is made towards the formation of a trachea, namely, by the branchial framework or skeleton, which opens into the mouth, and corresponds therefore properly to the larynx. Fishes are therefore the first animals which breathe through the mouth. In all the preceding classes the air entered the body, or the water found access to the branchiæ, by other routes.
3321. They may be termed Mouth-breathing animals, for the first formation of the trachea extends no further than to its junction or communication with the mouth; since, for it to be continued into the head and open self-substantially as a nasal organ, constitutes a second step in advance, which cannot be ventured upon in an Abdominal animal. In Fishes everything relates to the abdomen, and this is expressed by the first union of the trachea having taken place with the pharynx or the mouth.