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 
now, large and perfect shells are rare. The Atlantidce contains two genera, 
Atlanta and Oxygyrus. 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, 
Oxygyrus 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 
Atlantaperoni (magnified). 
