xciv Introduction. 



importance, but its various divisions, propodium, mesopodium, and 

 metapodium, may sometimes be all recognizable in the interspace 

 between the epipodial alae. The processes corresponding to the 

 propodium may be, like the arms of the Cephalopoda, armed with 

 suckers ; and these latter structures may be, as in Clione Borealis, 

 set in great niunbers upon certain circumoral retractile upgrowths, 

 which may correspond both to the acetabuliferous arms, and to the 

 buccal membrane, (itself also sometimes, as in LoUgo, acetabuliferous 

 in members) of that highest class of Mollusca. The Pteropoda may 

 further resemble the Cephalopoda by having, as in Cleodora, their 

 ordinarily large mantle cavity opening on the ventral or neural sur- 

 face. This cavity, however, may open upon the dorsal surface, as in 

 Gasteropoda, and it may be absent altogether, as in Clione. The 

 heart consists of a ventricle and auricle, and gives off an anterior 

 aorta which passes forwards through the nerve-collar to supply the 

 epipodial swimming organs. The small size of their bodies enables 

 them to dispense in most cases with branchial organs, both in the 

 families provided with a shell {Thecosomata), and in those destitute 

 of it {Gijmnosomata). Their renal organ has the normal internal 

 communication with the pericardial blood-sinus, as well as an opening 

 on to the exterior, but its walls may be either merely hyaline, or con- 

 tractile, without secretory tissue ; or thirdly, spongy and glandular, 

 as in the conchiferous families Hyalea and Cleodora. The eyes and, 

 usually, the sensory tentacles are more or less rudimentary. Auditory 

 vesicles are always present, and in relation with the pedal ganglia. 



The Pteropoda are hermaphrodite. The embryos go through a 

 metamorphosis, being provided, when set free from the egg, with 

 a bilobed ciliated veliun, which is replaced by the epipodia. The 

 Gymnosomatous Clionidae and Pneumodermidae go through a second 

 stage of metamorphosis, in which they have three zones of cilia. 



Most of the points of degradation, or simplicity, observable in the 

 structural arrangements of the Pteropoda, apj)ear to be referrible 

 either to their nocturnal habits, which have entailed a stunting of 

 the cephalic organs, or to their minute size, which has rendered any 

 complex evolution of the circulatory and respiratory organs unneces- 

 sary. They have frequently been classed as an order of Gasteropoda, 

 but the general relations of their motor organs and their mantle 

 cavity appear to approximate them rather to the Cephalopoda, with- 

 out however justifying us in ranking them as an order of that class. 



