394 



MOLLUSCS. 



only serves the purpose of protecting the vital organs, and in some forms it is 

 altogether wanting. The Heteropods may be divided into two families. Ptero- 

 tracheidce and Atlantidce. The former includes the genera Pterotrachea, 

 Firoloida, Cardiapoda, Carinaria, and Pterosoma ; a species of the typical genus 

 being shown on the preceding page. It is a transparent gelatinous creature. The 

 gills are exposed near the tail, and the fin-like foot, with the minute sucker upon 

 the edge, is seen on the opposite side. The sucker is smaller in the females than 

 the males. About sixteen species are more or less determinable. Firoloida 

 has no gill and the visceral nucleus is situated at the posterior end of 

 the body, with scarcely any caudal prolongation beyond it. The males are 

 provided with two slender tentacles in front of the eyes, the females being without 



these appendages, and the fin -sucker, 

 as in Pterotrachea, is also larger in 

 the males. Neither this genus nor 

 the preceding has any shell. In the 

 allied Cardiapoda the nucleus is 

 pedunculated, and partly protected 

 by a minute glassy spiral, shell. The 

 most interesting of this family is 

 Carinaria, on account of its beauti- 

 ful vitreous cap-shaped shell ; the 

 animal being rather like that of 

 Cardiopoda. The commonest species 

 is the well-known Mediterranean 

 C. lamarcki, but the largest is C. 

 cristata from the Indian and China 

 seas. The embryonic shell of 

 Carinaria is spirally coiled like a 

 snail-shell, and bears no resemblance 

 to the beautiful adult structure. The 

 latter at one time was so rare that 

 one hundred pounds is said to have 

 been given for a specimen. Even 

 The Atlantido3 contains two genera, 

 Atlanta and Oxy gyrus. The shells are small, compressed, and spirally coiled, of a 

 glassy texture, and capable of containing the animal. The gills are situated in a 

 dorsal cavity of the mantle, and the foot is trilobed, one portion of it supporting 

 a minute, subtrigonal operculum. About twenty species are recognisable, 

 Oxy gyrus being represented by only two. They abound in the warmer parts 

 of the Atlantic, Indian, and Pacific Oceans. 



Suborder Scutibranchiata. 



The great feature of these animals is the absence of certain functional 

 organs in the different sexes ; the radula being also of a peculiar type, and armed 

 with several central and lateral teeth. The two sections of the suborder, 

 Rhipidoglossa and Docoglossa, are based upon differences in the radula. In the 



Atlanta peroni (magnified). 



now, large and perfect shells are rare. 



