MOLLUSC A 20S 



Thus, in this animal at least, and probably in all MoUuscs, 

 the cavity of the generative organs and of the glandular part 

 of the kidney is part of the coelom, and the generative cells 

 are formed from coelomic epithelial cells. 



Branch B. EUTHYNEUEA. 



Chabacteeistics. — The, second great branch into which the 

 Gasteropoda Anisopleura is divided differs from the Strepto- 

 neura in the fact that the torsion of the visceral hump has 

 proceeded to a less extent, and that consequently the peri- 

 cardium is placed obliquely in front of the mantle cavity and 

 gill. For the same reason also the one side of the visceral loop 

 has not become pulled over the other by the forward rotation 

 of the gill and osphradium, and the loop is untwisted. The 

 condition of the gill and hidneys is as in Azygobranchiata. 

 The right auricle is wanting. With one exception, no 

 operculum is found. All Euthyneura are hermaphrodite, a 

 sharp distinction from the Streptoneura. The shell is often 

 but slightly calcified, and it and the mantle tend to disappear. 

 The Euthyneura comprise two orders : (i.) the Opistho- 

 branchiata and (ii.) the Pulmonata. 



The Opisthobranchiata are characterised, as their name 

 implies, by the position of the ctenidium behind the heart. 

 The branchial vein opens into the auricle, which is situated 

 behind the ventricle. The shell is often absent, and when it is 

 developed it is frequently covered by a fold of the mantle. 

 Both the mantle and the ctenidium may, like the shell, be 

 wanting, the function of the ctenidium may be carried on by 

 processes of the body-wall. 



In some Opisthobranchs the margin of the foot is produced 

 into two broad flaps, which may stand out at right angles to 

 the axis of the foot, or may in some cases be folded up against 

 the sides of the body. 



One of the most characteristic features of this order is the 

 presence of processes of the body-wall termed cerata or dorsal 

 papillae. The cerata vary very much in size, number, and 

 arrangement. Processes of the liver are prolonged into some 

 of them, these have been termed hepatoeerata to distingusih 



