298 INVERTEBRATE MORPHOLOGY. 



however, are developed iii many forms, the most important 

 being those connected with the foot and serving for locomotion, 

 retractor muscles in connection with the head, proboscis, and 

 tentacles, and the spindle-muscle, which has a general vertical 

 direction running along the right side of the visceral hump 

 from its insertion into the shell to the foot in whose wall its 

 fibres spread out, interlacing as it were with the horizontal 

 and transverse muscles there developed ; it serves to retract 

 the entire animal within the shell, and its development is 

 naturally in proportion to that of the shell, those forms in 

 which the shell is rudimentary or absent frequently lacking it. 



The euterocoelic portion of the coalom is much reduced 

 in comparison with what occurs in the Amphiueura, being dis- 

 tinctly represented only by a comparatively small pericar- 

 dium surrounding the heart, the auricle in some cases not 

 being enclosed by it. From analogy with the Amphiueura, 

 however, the reproductive organs must be regarded as repre- 

 senting a portion of the enteroccel whose connection with the 

 pericardium has been completely severed. A glandular struc- 

 ture, the pericardial gland, is in some Gasteropods developed 

 by the folding of the pericardial walls, and has apparently 

 an excretory function acting as an accessory nephridium ; it 

 is not, however, as highly developed as in some of the other 

 Molluscan groups. 



The circulatory organ possesses in some forms the charac- 

 teristic Molluscau structure, consisting of an unpaired ven- 

 tricle lying in the pericardium and receiving the blood from 

 two lateral wing-like auricles. In many cases, however, as al- 

 ready pointed out, the asymmetry produced by the develop- 

 ment of the visceral hump affects the heart, resulting in the 

 suppression of one of the auricles, that of the left (or right) 

 side (Fig. 133). In such cases the persisting auricle may 

 secondarily assume a terminal position with regard to the 

 ventricle, and the latter, instead of being continued into an ar- 

 tery at either extremity, gives off a single artery at the end op- 

 posite to that at which the blood enters from the auricle, this 

 artery dividing into two main trunks which distribute the 

 blood to the various regions of the body. These arteries may 

 be continued as distinct tubes with definite walls for some 



