THE DIFFEBENTIATION OF THE MESODEBM-BUDIMENT, ETC. 209 



may be Found at an earlier period on the surface of the body, as in 

 Fasciolaria (Osbobn, No. 81). 



Bipectinate plumose gills, a pair of which is found in Fissurella and Haliotis, 

 are considered as the mosl primitive, and we may assume that the single 

 monopectinate gill of the Monotocardia is to he derived from these, one of 

 the gills (originally the left) disappearing through the shifting of the pallial 

 complex while the other (originally the right), by fusion with the inner wall 

 of the mantle-cavity, loses one of the rows of its leaflets.* So little attention 

 has as yet been bestowed on I be development of the gills in the Gastropoda 

 that it is impossible to confirm by their ontogeny this view which in any 

 case is very probable. The derivation of the single gill from the double gill is 

 also plausible because the former is found not only in the most primitive 

 Gastropods, but also in the Amphineura, the lowest Lamellibranchs and the 

 Cephalopoda, i.e. in all the principal divisions of the Mollusca. 



G. The Differentiation of the Mesoderm-rudiment, the 



Development of the Body-cavity, the Nephridial 



and Circulatory Systems. 



Apart from the primitive kidneys (pp. 136, 178) little has yet 

 been recorded of the formation of the mesodermal organs. We have 

 already shown that the mesoderm appears as a bilateral rudiment 



which is soon found in the form of two cell-masses, comparable to 

 the mesoderm-bands of the Annelida, at the posterior end of the 

 body near the blastopore (Figs. '.Mi, 48, 51, 52, 56). The distinctness 

 of these two cell-masses varies in the different forms; they may also 

 he considerably reduced in size at an early stage, single cells being 

 detached from them and becoming distributed in the primary body- 

 cavity. By the development of a cavity in each of these cell-masses, 

 right and left coelomic sacs are formed (Fig. 56 A and C), in which 

 a somatic and a splanchnic layer can be distinguished. As a rule, 

 however, this process is not so simple as that described for Bffthinia 

 by v. Erlangee. The detachment of the cells from the two masses 

 usually occurs very quickly, the two coelomic sacs being then much 

 more difficult to recognise. They represent, in the main, the rudi- 



*[ln Trochus, the septum, which separates the two sets of leaves of the 

 single gill, is attached (except al the free end) to the mantle-wall along ; bol h 

 its margins; in this way one set of gill-leaves becomes enclosed in a small 

 cavity which only communicates with the general pallial cavity in front. 

 These gill-leaves arc much reduced in size as compared with the set which 

 project into the main mantle-cavity, and it is easy to see that a further stage 

 in this process might resull in a complete fusion of the septum with the 

 mantle-wall and thus cause a suppression of the one set of gill-leaves. There 

 is every reason to believe thai the monopectinate -ill arose in this way. — Ed. 



I' 



