586 THE ANATOMY OF INVERTEBRATED ANIMALS. 



of these is still a matter of doubt, but in many cases it ap- 

 pears to be unquestionable that they are derived from the 

 hypoblast. 



The perienteron, more or less interrupted and broken up 

 by the constituents of the mesoblast, may give rise directly to 

 the perivisceral space, or channels, of the adult, which thus 

 constitute a schizocoele. It is hardly doubtful, I think, that 

 the perivisceral cavity takes its origin in this manner in the 

 Rotifera, the entoproctous Polyzoa, the Echinopaedia of the 

 Echinoderms, the Tunicata, and the Nematoidea. 



On the other hand, in many Tnvertebralct, one or more di- 

 verticula of the archenteron extend into the perienteron and 

 its contained mesoblast. Sometimes, as in the Ccelenterata, 

 these remain connected with the alimentary cavity through- 

 out life, and are termed gastrovascular canals. In other cases 

 (Echinodermata, Brachiopoda, Chcetognatha) they become 

 shut off; their cavities constitute a variously -modified enter o- 

 eoele y and their walls give rise, along with the primitive 

 mesoblastic elements, to the mesoderm. 



To which of these two possible sources of the mesoderm, 

 the mesodermal structures of the Annelida and the Arthro- 

 poda, which so very generally take on the form of two longi- 

 tudinal germ-bands in the embryo, and subsequently undergo 

 segmentation, are to be referred, is a very interesting, but, as 

 yet, unsolved problem. It is possible that they are solid rep- 

 resentatives of the hollow diverticula which, in other animals, 

 give rise to the enteroccele ; in which case the perivisceral 

 cavity in these animals will be a virtual enteroccele. On the 

 other hand, they may merely represent the cells of the meso- 

 blast of the entoproctous Polyzoa and of the Echinopasdia, 

 and their perivisceral cavity would then be a schizocoele. But 

 it is needless to pursue this topic further ; enough has been 

 said to show conclusivelv that, however different one inver- 

 tebrated animal may be from another, the study of develop- 

 ment proves that each, when traced back through its embry- 

 onic states, approaches the earlier stages of all the rest ; or, 

 in other words, that all start from a common morphological 

 type, and even in their extremest divergence retain traces of 

 their primitive unity. 



It is very important to remark that these morphological 

 generalizations, so far as they are correctly made, are simple 

 statements of fact, and have nothing to do with any specula- 

 tions respecting the manner in which the invertebrated ani- 



