518 



MOLLUSC A. 



TlIK SAi.l I I \. 



THE SAGITTID^. 

 These iiro iittlc fisli-likc animals funiislRMl with one or two fin-like organs on the body and a 



broad and usually bilobcd caudal-fin. 

 They are of small size and swim with 

 great rapidity. They liave hitlierto been 

 found principally in the North Sea and in 

 the Mediterranean. The name of Sagitta, 



given to these animals, refers to tlieir arrow-like appearance. 



THE FIROLIDiE. 



These are cither entirely naked or furnished 

 — as is the case with the Carinaria cijmhlum, of 

 which an engraving is annexed — with a small, 

 conical, keeled shell, which incloses the intes- 

 tinal nucleus. The Carinaria has an elongated 

 transparent body, dotted with elevated points, 

 and fui'nished toward the upper part of the 

 posterior extremity with a sort of fin, which 

 performs the office of rudder ; nearly opposite 

 to this, on the belly, is a semicircular fin ; with the aid of this it floats in the water. The 

 shells of this genus were formerly known under the head of Venus' Slipper and the Glass Nau- 

 tilus. The C. Mediterranea is abundant in the vicinity 

 of Nice. A few other species are found in warm climates. 



THE ATLANTID^. 



In these the shell is spiral, and so large as to contain 

 the whole animal when it is contracted. These were 

 formerly supposed by some naturalists to be the original 

 of the fossil ammonites, or at least an analogous family. 

 In the Atlanta Peronii, found in the vicinity of the Canary 

 Isles, the shell is minute, glassy, and compressed. In the 

 THE ATLANTA PERONII. auuexcd cugravlng, the figure a shows the actual size. 



THE CAIUXAKIA, SWIMMING Willi HACK AM) SHELL DOWNWARD. 



Cla§s III. fTE IiOPOI>A. 



This term is derived from the Greek j^i^ron, wing, and pous, foot, and is descriptive of these 

 little animals, which are furnished with a pair of broad, flattened fins at the sides of the head, 

 by means of which they are able to swim with tolerable rapidity, through the open sea, which 

 is their favorite abode. They seldom approach the shores unless driven thither by the winds. 

 They often crowd the sea in such inconceivable numbers as to color the surface for many miles. 

 There are two orders of pteropoda characterized by the presence or absence of a shell. 



ORDER 1. THECOSOMATA. 



The animals of this order are always inclosed in a shell, usually very delicate and of a glassy trans- 

 parency. There are several genera, 

 *'3ps-^ ^Sf^ *^^ forms differing greatly from each 



^' _-.* ^ .V ..—. other. The sliell of the Clcodora 



pi/raiuidata is triangular; that of the 

 Limacina rostralis resembles a small 

 nautilus in form. The Hyalea tri- 

 dentata is globular and transparent, 

 Avith long appendages to the mantle. 

 This is common in the Atlantic, 

 cLtoDORA PTRAMiDATA. LiMAciNA ROSTRALIS. HTALEA TRiDENTATA. Mediterranean, ludlau OccaH, ifec. 



