MOLLUSCS. 



357 



respiratory organs, and thence to the heart. In some forms no respii-atory organs are 

 present, the skin serving the purpose, but in others gills occur, none of which (excejit 

 possibly those of Pneumodermon) are the homologues of those of most molluscs. In 

 Pneumodermon the gills ai-e borne on the hinder end of the body, but in the theco- 

 somatous forms there are folds in the mantle cavity which subserve respiratory ])ur- 

 poses. The kidneys are long, contractile tubes, communicating internally with the peri- 

 cardial cavity, and externally either with the mantle cavity, or with the exterior. In 

 nervous system the ptei-opods are very similar to the opistliobranchs. 

 The cerebral ganglia in one group are nearly united and placed above 

 the throat, but in others they occupy widely separated, lateral posi- 

 tions. Two auditory capsules are present, placed near the pedal 

 ganglia, but eyes are either absent or remain in a very rudimentai-y 

 condition, a fact which accords with their nocturnal habits. 



The pteropods are hermaphroditic, the genital glands being placed 

 beneath the heart in the visceral sac, and commonly possessing a 

 common duct. The eggs are laid in long cylindrical strings, which 

 float freely about in the sea. The development of some of the sitecies 

 has been followed by the eminent Swiss embryologist, Henuann Fol. 

 The larva is provided with a velum and a shell, the latter cajjable of 

 being closed by an operculum. With growth, the embryonic shell 

 is cast aside, and in some is replaced by a second permanent shell, 

 while others remain naked throurdi life. This presence or absence Fio- "i;- — ''"'itrio 



» I columella. 



of a shell in the adult is correllated with other structural peculiarities 

 which served, in the hands of De Blainville, to divide the sub-class into two orders. 

 The pteropods are mostly small, but few acquiring a length of an incii or more ; 

 yet, notwithstanding their small size, they are objects of beauty. They live upon the 

 high seas in all quarters of the globe, at times appearing in vast multitudes, especially 

 at the approach of darkness and during the later moments of twilight. They swim 

 about l)y the violent flappings of their wings, or they retract these organs and sink at 

 will into the depths of the sea, out of the reach of storms. 



Order I. — THECOSOMATA. 



As the name implies, this division of the pteropods has the body enclosed in a 

 case or shell, and with this protection of the body are associated other structural 

 features, which need to be only briefly alluded to. The head is but feebly developed ; 

 indeed, is frequently not distinct from the rudimentary foot which remains in con- 

 nection with the wings or gi-eatly developed epipodia. Rudimentary tentacles are 

 present. 



In the IIYALEID.E the shell is horny or calcareous, never spiral, but either nearly 

 globular or needle-shaped, the two sides being symmetrical, temiinating with one or 

 three sharp points. The cavity of the mantle opens ventrally and contains a horse- 

 shoe-shaped plaited and ciliated gill. Best known in the family is the genus IJyalea, 

 or, as it is frequently termed, Cavolina, the species of which belong to temperate or 

 tro])ical seas. The shell is nearly globular, somewhat flattened above and rounded 

 below, while the posterior extremity terminates in three points. The aperture is 

 narrow and is continued by a slit on either side, through which are protruded folds of 

 the mantle (shown in our figure), the function of which is uncertain. The Hijalece 



