Characteristics of Pteropoda. xciii 



except the Entoconcha and Rhodope ; it is sometimes^ as in Chiton, 

 Ner'dhia, HaUotis, perforated by the rectum, as is the case in the 

 Lamellibranchiata, to which some of these Gasteropoda fm-nish an 

 additional point of resemljlance in possessing two auricles. In many, 

 though probably not in all Gasteropoda, the perivisceral cavity is in 

 direct communication with the blood-vascular system. There may 

 be no specialized organ of respiration ; gills, however, are ordinarily 

 present, except in the Pidmonata, where atmospheric air is inhaled 

 into a cavity formed by the mantle. The renal organ is single. 



The Gasteropoda may be either dioecious or hermaphrodite. In 

 a few dioecious species which are, as Vermetus and Siliquaria, fixed 

 to one spot, there is no sexual congress, but the ova are fertilized 

 by the spermatozoa finding their way to them after being set free 

 into the water ; and in a few of the hermaphrodite species, such as 

 Terg'qies Edwardsi and Limnaea auricularis, heavitandrous impreg- 

 nation has been observed to take place. But with these exceptions, 

 sexual congress always precedes impregnation, and indeed all re- 

 production, in Gasteropoda. The accessory reproductive apparatus 

 is greatly developed and complex in the hermaphrodite orders, Pul- 

 monata and NudibrancJiiata, whilst in some of the dioecious orders, 

 Cj/clohranchiata and Aspidobranchiata, even the intromittent organ 

 may be wanting, and microscopic examination may be necessary for 

 the distinguishing of the sexes. Gasteropoda are all but universally 

 oviparous, the yolk undergoing segmentation, and manifesting the 

 phaenomenon of rotation whilst within the egg. When the embrj^o 

 is set free from the egg, it ordinarily goes through a metamorphosis 

 which is marked by the possession of a provisional organ in the shape 

 of a bilobed ciliated locomotor velum. The embryos of the Pid- 

 wonata, in which order the ova may attain a very great size, may 

 possess from the first the form and organization of the adult animal, 

 but provisional organs have been observed in their development 

 [Llmax) as in that of Brauchiorjasteropjoda. 



Class, Pteropoda. 



Mollusca of small size varying from i"' to 3" in length, of pelagic 

 habitat, of nocturnal habits, with the head and eyes rudimentary, 

 and with the epipodia largely developed and constituting swimming 

 organs. The foot proper is ordinarily much reduced in size and 



