THE ASCIDIOIDA. 



31 



dians which I have examined, the first bend of the intestine 

 takes place in such a manner that ? 

 if the intestine continued to preserve 

 the same direction, it would end on 

 the opposite side of the mouth to the 

 nervous ganglion (Fig. 12); in other 

 words, the nervous ganglion would 

 not be situated in the re-entering 

 angle between the gullet and the 

 rectum, but on the opposite side of 

 the gullet to that angle. Therefore, 

 the flexure of the intestine is not 

 neural, as in the Potyzoa ; but as, on 

 the contrary, the intestine is pri- 

 marily bent towards the heart-side of 

 the body, its flexure may be termed 

 "haemal." And this haemal flexure 

 of the intestine in the Ascidiaus thus 

 constitutes an important element in 

 the definition of the class. 



In these animals there is an atrial 

 system, the development of which is 

 even more extraordinary than in the 

 Polyzoa. The second aperture to 

 which I have referred (&, Fig. 11 and 

 I, Fig. 12) is continued into a large Kg. i 2 .- 

 cavity, lined by a membrane, which 

 is reflected, like a serous sac, on the 

 viscera, and constitutes what is called 

 the "third tunic," or "peritoneum." 

 From the chamber which lies imme- 

 diately within the second aperture 

 (k, Fig. 12) it is reflected over both 

 sides of the pharynx, extending, to- 

 wards its dorsal part, very nearly as 

 far as that structure which has been 

 termed the " endostyle " (m, Fig. 12). 

 It then passes from the sides of the 



mentula; the 

 test removed, and hardly more 

 of the animal drawn than 

 would be seen in a longi- 

 tudinal section, a, oral aper- 

 ture ; I, atrial apei ture ; c, 

 circlet of tentacles ; d, pha- 

 ryngeal, or branchial, sac : the 

 three rows of apertures in its 

 upper part indicate, but do not 

 represent, the pharyngo-atrial 

 apertures ; e, the languets : a 

 series of tongue-shaped pro- 

 cesses which project into the 

 bi-anchial sac ; /, oesophageal 

 opening; g, stomach; h, in- 

 testine performing its haemal 

 flexure ; i, anus ; k, atrium ; 

 6, ganglion ; m, endostyle ; , 

 heart. 



