200 



PTEBOPODA. 



13 



FIG. 109. FREE SWIMMING PNEUMODERMON 

 LARVJE. (After Gegenbaur, copied from BroDn.) 



The velum has atrophied iu both larvae. 



In A three ciliated bauds are present, aud the 

 auditory vesicles are visible. 



In i> the tentacles with suckers aud the epi- 

 podia have become developed. 

 an. anus. 



itself from the larval shell, which becomes shut oft' from the permanent 

 shell by a diaphragm. The larval shell then becomes detached. 



In the Styliolidse the per- 

 manent shell becomes twice 

 the size of the embryonic 

 shell while the animal is still 

 in an embryonic condition, 

 but the larval shell persists for 

 life. In the Cymbulidae there 

 is an embryonic and secondary 

 shell, which persist together 

 during larval life. They are 

 eventually cast oft" at the same 

 time and replaced by a per- 

 manent shell. 



In the Gymnosomata an 

 embryonic shell is developed, 

 and a secondary shell added 

 to it during embryonic life. 

 Both are cast off" before the 

 adult condition is attained. 

 After the shell has been cast 

 off three ciliated rings are de- 

 veloped (tig. 109). The an- 

 terior of these is placed between the velum and the foot, and the two 

 hinder ones on the elongated posterior part of the body. 



The ciliated rings give to these larvae a resemblance to Chsetopod Iarva3 ; 

 but there can be no doubt that this resemblance is a purely superficial one. 

 The anterior ring atrophies early (fig. 109 B), and the second one soon 

 follows suit. It is probable that the hindermost one does not persist 

 through life, although it has been observed in forms with fully developed 

 sexual organs. Most of these larvte have not been traced to their adult 

 forms. They have been referred to Piieumodermon, Clio, etc. 



The most characteristic organ of the Pteropods is the foot, which is 

 prolonged into two enormous lateral wings, the epipodia. These develope 

 at different periods in different larvae, but are always distinct lateral out- 

 growths of the foot. 



In the Hyaleidpe the foot is early conspicuous, and soon sends out two 

 lateral prolongations (fig. 108 pn.} which develope with enormous rapidity 

 as compared with the medium portion, and give rise to the epipodia. The 

 whole of the foot becomes ciliated. 



In the Cymbulidfe, though not in other forms, an operculum is developed 

 on the hinder surface of the foot (fig. 103 C). The epipodia are late in 

 appearing. 



In the Gymnosomata the foot is developed very early, but remains 

 small. The epipodia do not appear till very late in larval life (fig. 109 B). 



In Piieumodermon and some other Gymnosomata there appear on the 

 hinder part of the head peculiar tentacles with suckers like those of the 

 Cephalopoda (fig. 109 B). It is not certain that these tentacles are geneti- 

 cally related to the arms of the Cephalopoda. 



Cephalopoda. The eggs of the Cephalopoda are usually laid in 



