588 COMPARATIVE ANATOMY chap. 



The body lengthens and its preoial section becomes produced into the proboscis, 

 at whose tip the eyes are still long visible, until they degenerate together with the 

 apical plate and tuft. 



The preoral and jjostoral ciliated rings degenerate, but the whole body becomes 

 covered with cilia. 



The principal ciliated ring (Fig. 467) persists for some time, eventually, i.e. 

 when the anal area has increased in length, surrounding the body about half 

 way between the mouth and anus. A circular furrow between it and the base 

 of the proboscis is the first indication of tlie i)osterior boundary of the collar 

 region. 



The whole ectoderm of the oral area degenerates during metamorphosis, the 

 body epithelium proceeding exclusively from the ectoderm of the preoral, postoral, 

 and anal areas, which increases in thickness. Tliis phcndincndn, together witli tlie 

 lengthening of the larva, causes a very marked diminution in the transverse section 

 of the body, an essential accompaniment of which is the approximation of the 

 larval integument to the parietal walls of the ccelomic vesicles. 



2. Anatomical. — The apical plate of the Tornaria larva, which completely 

 degenerates later, consists of a dorsal and a ventral half. Below tlie surface of the 

 specially thick dor.sal half there is a layer of nerve fibres. 



Tlie centre of the apical plate is formed by a small group of long narrow 

 sensory cells carrying delicate immobile cilia (sensory hairs). 



The two eyes which are embedded in tlu> apical plate are optic pits, whose 

 floors are formed by cells which are pigmented at their bases (retinal cells ?).^ The 

 optic pit is filled witli a clear substance, which is a continuation of the cuticle 

 covering the apical i)late, and is called the lens. The apertures of the two pits 

 diverge anteriorly and laterally. Round them in the apieal plate thee is a 

 deeper layer of elements, which are considered to be ganglion cells. 



The alimentary canal. — Even the youngest larv;e observed showed tlie division 

 of the alimentary canal into oesophagus, mid-gut or stomach, and hind-gut, which 

 is characteristic of all the larval stages. 



The oesophagus ascends vertically. It is a flattened tube provided witli a 

 circular musculature, its thickened dorsal and ventral walls being ciliated. 



The stomach is a large egg-shaped sac, whose axis lies horizontally, and into 

 whose anterior pole the n?sophagus enters. The epithelial cells of the sac are at 

 first low, but they lengthen at a later stage. The originally thin-.skinned stomach 

 develops in this way a thick w^all, which is probably non-ciliated except at two 

 points. A ciliated cushion is found ventrally, at tlie entrance to the stomach, and 

 the efferent ajierture is surrounded by long hairs, which perhaps act as a fish-trap 

 apparatus. 



The hind-gut, in the youngest larviie, is an almost cylindrical tul)c witli tliin 

 walls. At a later stage its anterior jiart becomes swelled up, so that the hind-gut 

 as a whole becomes funnel-shaped or conical. As, however, the aperture com- 

 municating with the stomach remains small, the wall of the hind-gut is applied 

 over a considerable area to the jiosterior wall of the stomach. The aperture lies at 

 the centre of this area. Immediately in front of the anal aperture there is a circle 

 of cells provided with cilia. 



Formation of the gills. — The formation of the first pair of gills takes place 

 shortly l)efore or after nietamori)hosis. Two lateral caeca arise at the posterior end 

 of the resoi)hagus, grow out towards the integument, and break out laterally and 

 dorsally through the external branchial pores. The oesophageal ai)ertures of these 

 branchial diverticula are at first round, later they become U-shaped, by the lateral 

 dorsal intestinal wall growing out into the diverticulum. This outgrowth is the 

 rudiment of the branchial tongue. 



