PTEROPODA. 203 



the water is discoloured for leagues {Scoresby). They are seen swimming 

 at the surface in the heat of the day, as well as in the cool of the evening. 

 Some of the larger kinds have prehensile tentacles, and their mouths armed 

 with lingual teeth, so that, fragile as they are, they probably feed upon still 

 smaller and feebler creatures, {e.g. entomostraca) . In high latitudes they 

 are the principal food of the whale, and of many sea-birds. Their shells arc 

 rarely drifted on shore, but abound in the fine sediment brought up by the 

 dredge from great depths. A few species occur in the tertiary strata of Eng- 

 land and the continent ; in the older rocks they are unknown, unless some 

 comparatively gigantic forms {conularia and theca) have been rightly referred 

 to this order. 



In structure, the Pteropoda are most nearly related to the marine uni- 

 valves, but much inferior to them. Their nervous ganglia are concentrated 

 into a mass ieloio the oesophagus ; they have auditory vesicles, containing 

 otolites ; and are sensible of light and heat and probably of odours, although 

 at most they possess very imperfect eyes and tentacles. The true foot is 

 small or obsolete; in cleodora it is combined with the fins, but in Clio it is 

 sufficiently distinct, and consists of two elements ; in Spinalis the posterior 

 portion of the foot supports an opercuhim. The fins are developed from the 

 sides of the mouth or neck, and are the equivalents of the side-lappets 

 {epipodia) of the sea-snails. The mouth ol Pneumodermon\?,i\ivnh\itdi with 

 two tentacles supporting miniature suckers ; these organs have been compared 

 with the dorsal arms of the cuttle-fishes, but it is doubtful whether their 

 nature is the same.* A more certain point of resemblance is the ventral 

 flexure of the alimentary canal, which terminates on the under surface, near 

 the right side of the neck. The pteropods have a muscular gizzard, armed 

 with gastric teeth ; a liver ; a pyloric coecum ; and a contractile renal organ 

 opening into the cavity of the mantle. The heart consists of an auricle and 

 a ventricle, and is essentially opistho-branchiate, although sometimes affected 

 by the general flexure of the body. The venous system is extremely incom- 

 plete. The respiratory organ, which is little more than a ciliated surface, is 

 either situated at the extremity of the body and unprotected by a mantle, or 

 included in a branchial chamber with an opening in front. The shell, when 

 present, is symmetrical, glassy, and translucent, consisting of a dorsal and a 

 Ventral plate united, with an anterior opening for the head, lateral slits for 

 long filiform processes of the mantle, and terminated behind in one or three 

 points ; in other cases it is conical, or spirally coiled and closed by a spiral 

 operculum. The sexes are united, and the orifices situated on the right side 

 of the neck. According to Vogt, the embryo Pteropod has deciduous vela, 



* The figures of Eydoux and Souleyet represent them as being supplied with 

 neives from the cephalic ganglia ; whereas the arms of the cuttle-fisli, and all other 

 parts or modifications of the foot, in the mollusca, derive their nerves from the pedal 

 ganglia (Huxley). 



