ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



PTEROPODA. 



53 



A 



B 



4^ 



S^/fr 



D 



Fig. 39 Median and lateral teeth of A: Pneumodermopsis macrochira; B. Pneumodermopsis michaelsarsi : 



C. Pncumoderma sp, (young); D, Pneumoderma atlantica. 



All drawn to the same scale. 



The whole appearance of the animal varies very much 

 according to the degree of contraction. In most of my 

 specimens the anterior as well as the posterior organs of 

 the body were more or less completely concealed within 

 the middle portion, so that these individuals look like little 

 spheres from 3 to 6 mm. in diameter. In others (pi. V, 

 fig. 37 — 38) the foot and wings and posterior gill are 

 more or less clearly visible, while in one individual (fig. 

 35 — 36) the buccal apparatus is also partly evaginated 

 and the acetabuliferous appendages extended. This great 

 difference in the outward appearance has made it necessary 



to investigate the buccal apparatus in several individuals 

 in order to determine their identity. 



The formula of the radula is 6—1—6, the size of 

 the lateral teeth gradually increasing towards the median 

 line (textfig. 40 B, C). In full-grown specimens (B) they 

 are fixed to large basal plates, which however are not 

 found in younger specimens (C). 



The jaw is a conical papilla (textfig. 41 and pi. V. 

 fig. 41) covered with spines of different sizes. The larger 

 spines seem in some specimens to be arranged especially 

 along three ridges meeting at the top of the cone, while 



