PTEROPODA 435 



other four, while some have held them to be a subdivision of 

 Cephalopoda. Modern authorities, chief among whom is Pel- 

 seneer, regard the Pteropoda not as a primitive, but as a derived 

 and recent group. They are " Gasteropoda in which the adapta- 

 tion to pelagic life has so modified their external characters as 

 to give them an apparent symmetry." 



The principal point which relates the Pteropoda to the 

 Gasteropoda is the asymmetry of the visceral organs, intestine, 

 heart, kidney, and genital gland, which results from their 

 development on one side only of the body. Their hermaphro- 

 ditism and the structure of their nervous system relate them to 

 the Euthyneura rather than to the Streptoneura. Resemblances 

 in the organs of circulation and generation approximate them to 

 the Opisthobranchiata rather than to the Pulmonata, while of 

 the two groups of the former, they tend to closer relationship 

 with the Tectibranchiata than with the Nudibranchiata. The 

 two sections of Pteropoda have been considered of distinct 

 origin, the Thecosomata being derived from the Bulloidea, the 

 Gymnosomata from the Aplysioidea.^ 



Thus the Pteropoda are a group whose true relations are 

 masked by the special conditions of their existence, which have 

 tended towards the development of certain organs, the so-called 

 " wings " and the shell, which give them an apparent symmetry ; 

 this symmetry disappears on a closer investigation of the internal 

 organs. They are hermaphrodite ; the genital gland has a single 

 efferent duct (except in some Oavolinia}, a seminal groove lead- 

 ing to the copulatory organ, which in the Thecosomata is on the 

 right side of the head, in the Gymnosomata on the right side of 

 the foot. The genital system resembles that of the Opistho- 

 branchiata and of the "digonoporous " Pulmonata. 



Section 1. Thecosomata. — Shell or cartilaginoid test al- 

 ways present, fins united by an intermediate lobe, ctenidia as a 

 rule absent, replaced by secondary branchiae, no very distinct 

 head or eyes, one pair of tentacles ; cerebral ganglia on the sides 

 of and under the oesophagus ; radula with three rather large 

 teeth in a row, generally unicuspid, jaw in two pieces, stomach, 

 with horny plates, anus generally on the left side. 



The Thecosomata feed on Protozoa and the lower Algae ; 



1 It appears more convenient to treat the whole group together, rather than 

 deal with the two sections separately. 



