THE ASCI 1)101 DA. 



:;i 



second aperture to which I have refen 

 continued into a large cavity, lined by 

 a membrane, which is reflected, like a 

 serous sac, on the viscera, and consti- 

 tutes what is called the " third tunic," 

 or k< peritoneum." From the chamber 

 which lies immediately within the 

 second aperture (k, Fig. 11) it is re- 

 flected over both sides of the pharynx, 

 extending, towards its dorsal part, very 

 nearly as far as that structure which 

 has been termed the "endostyle' : 

 (m, Fig. 11). It then passes from the 

 sides of the pharynx to the body walls, 

 on which the right and left lamellae 

 become continuous, so as to form the 

 lining of the chamber (h), into which 

 the second aperture (b) leads, or the 

 " atrial chamber." Posteriorly, or at 

 the opposite end of the atrial chamber 

 to its aperture, its lining membrane 

 (the " atrial tunic ") is reflected to a 

 greater or less extent over the intes- 

 tine and circulatory organs, some- 

 times inclosing each of their parts 

 in distinct plications (as in the genus 

 Phallusia), sometimes merely passing 

 over them, and limiting the blood 

 sinus in which they are contained 

 (as in Clavelina, &c). Where the 

 atrial tunic is reflected over the sides 

 of the pharynx, the two enter into 

 more or less close union, and the sur- 

 faces of contact become perforated by 

 larger or smaller, more or less nume- 

 rous, apertures. Thus the cavity of 

 the pharynx acquires a free commu- 

 nication with that of the atrium ; and, 



ed (fc,Figs. 10 and 11) is 

 Fig. 11. 

 a 



Fig. 



1 1 . — Phallusia mentula ; the 

 test removed, and haidly more 

 of the animal drawn than 

 would be seen in a longi- 

 tudinal section, a, oral aper- 

 ture ; b, atrial aperture ; 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 ; c, the languets : a 

 series of tongue-shaped pro* 

 cesses which project into the 

 branchial sac ; /, oesophageal 

 opening ; g, stomach ; h, in- 

 testine performing its haemal 

 flexure ; i, anus 

 /. ganglion 

 heart. 



h, atrium ; 

 m, endostyle ; n, 



