PTEROPODA. 



55 



which the arteries open. The blood returns from the lacuna? through 

 the respiratory organs to the pericardia! sinus, whence it enters the 

 auricle through the venous ostium. 



The respiratory organs, as far as they are not represented by the 

 whole integument (Clio], have the form either of foliaceous branchial 

 appendages (Pneumodermon) at the hind end of the body, or, in the 

 shell-bearing forms, of internal gills placed within the mantle cavity, 

 the entrance to which is lined with peculiar ciliated bands. The 

 gills are always but slightly developed, and are reduced either to 

 folded elevations of the ciliated mantle-wall, or to the mantle-wall 

 itself. 



The kidney is an elongated contractile sac, which communicates 

 with the pericardia! sinus by a ciliated 

 funnel, and with the mantle cavity or 

 directly with the exterior by a strongly 

 ciliated opening which is capable of 

 being closed. 



The nervous system resembles that 

 of the higher Opisthobranchs. Pleura! 

 ganglia are present. The cephalic cones 

 receive their nerves from the brain ; the 

 two fins as parts of the foot from the 

 pedal ganglia. 



Sense organs. A pair of auditory 

 vesicles are always present. Eyes on 

 the other hand are absent or very rudi- FlG - 528.-Larva of Cavoimia tride 



J tuta (after Fol). Ms, velurn ; ] 



mentary, as red pigment spots (Hyalea.) 

 placed either on the visceral sac' near 

 the oasophageal ring or on the tentacles 

 (Clio). Tactile organs are represented by two small tentacles 

 (Hyalea, Cymbulia] and the larger cephalic cones which are some- 

 times beset with suckers (Clio and Pneumodermon}. 



The Pteropoda are hermaphrodite. The hermaphrodite gland lies 

 near the heart behind the stomach in the visceral sac, and usually 

 possesses a common duct which is provided not only with a seminal 

 vesicle, but also with a kind of albumen gland and receptaculum 

 seminis; it opens to the exterior usually on the right side in front 

 of the anus. The penis is sometimes in the terminal part of the 

 duct ; in the Hyaleidce and Cymbuliidce it has the form of a rolled-up 

 protrusible tube placed in front of the sexual opening. The eggs are 

 surrounded by albumen and laid in long strings which float freely in 



foot; P. the two lateral (epipoilial 

 lobes of the foot; A, anus; 31. re- 

 tractor muscle ; Md, stomach. 



