526 Causes and Course of Organic Evolution 



in Povipholyx four elements — a dorsal, a ventral, and two 

 laterals — together make up the lorica. But in all of Hudson- 

 Gosse's accounts the dorsal shell seems to be the largest and 

 first formed. Now in lamellibranchs, as in several classes 

 already cited, a dorsal median shell is first formed, while in 

 lamellibranchs only the lateral shells arise as dorso-lateral 

 calcareous formations in connection with the highly typical 

 molluscan shell-gland. A phylogenetic indication of this 

 gland in rotifers would therefore be of interest. 



Now Gosse (169, II: 102) speaks as follows regarding Colurus 

 deflexus: "In the middle of the back, just under the lorica, are 

 two curious organs, each apparently an agglomeration of 

 minute clear vesicles, perhaps of air, perhaps of oil, observed 

 long ago by Ehrenberg. He declared them inexplicable; and 

 I cannot supply the explanation." x\gain (p. 104) Gosse 

 says for C. amblytelus, "one oil globule (sometimes two) occu- 

 pies in general the middle of the back, and is conspicuous." 

 And again he says for Metopidia lepadella (p. 106): "No organi- 

 zation was visible within, save two conspicuous clear vesicles, 

 side by side in the middle of the body, not at all like oil globules, 

 being irregularly oblong; nor accidental, being found in each 

 of a large number of individuals seen at different times." 



Though meager in detail, this account may next be compared 

 with those given for the shell-gland, and its relation to the 

 shell in lamellibranchs. "The shell, as in the Gasteropoda, 

 is unpaired in its origin, and is formed by a secretion of the 

 epithelium of the shell-gland. This embryonic cuticular shell 

 is retained, and passes over into the periostracum (epidermis) 

 of the adult shell. The latter arises through a secretion of 

 granular calcareous substance; this at first accumulates in 

 two complexes lying symmetrically at the two sides of the 

 body beneath the cuticular shell, which, by further increase, 

 yield the shell-valves. These grow out dorsally until they 

 meet. The dorsal part of the cuticular shell, that lies between 

 the two calcified shell valves, which are growing towards one 

 another, yields the ligament of the shells. The larval shell 

 which thus arises, and which has a very simple structure, is 

 retained in Ostrcea, the Unionidse, and, as has recently been 

 proved, in a number of other Lamellibranchs" (77i, /; 60). 



Further Ziegler has shown that "an accumulation of small 

 calcareous concretions can be seen on each side of the dorsal 

 middle line, beneath the unpaired cuticular integument. These 

 masses increase and soon take the form of the definitive shell. 

 There is now a shell-plate on each side; the two, however, are 

 not in contact, but are somewhat far removed from the middle 



