ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



391 



From the study of larvae of the various classes, the author concludes 

 that a pair of anterior entercoels was probably originally present in all 

 Echinoderms ; the hydrocoel is generally formed distinctly later than the 

 other cavities ; as there is no trace of any right hydrocoel, it is probable 

 that this organ was never paired ; it varies in its mode of origin, but 

 its normal position seems to be between the anterior and posterior 

 enterocoels. The water-pore always (? Holothurians) arises in connec- 

 tion with the posterior end of the left anterior enterocoel, and only 

 communicates indirectly, if at all, with the hydrocoel. Mr. Bury thinks 

 that the water-pore existed in a very early stage in the history of 

 Echinoderms, and j)robably before the hydroccel. 



The current passing through the water-pore is exhalent, but not 

 usually very strong ; this pore, and the short tube by which it com- 

 municates with the enterocoel, are regarded as forming a primitive 

 nephridium. 



The hydrocoel always arises on the left side from one or other 

 division of the coelom, which differs in different groups ; this variation 

 may mean that when the hydrocoel originally appeared, the anterior and 

 posterior enterocoels were connected, as they are in Asterina and some 

 Bipinnarise ; or it is possible that the original mode of development of 

 the hydrocoel was more complicated, and that it has become simplified 

 independently in the different groups. The author has a few observa- 

 tions on the difficult question of the closure of the water-vascular 

 ring. 



In the study of the skeleton it was found that many plates are 

 developed in the bilateral larva, and that they bear a definite relation to 

 the body-cavities ; it was also found that the terminals lie on the left 

 side, and this discovery enables us to establish a typical bilateral form 

 from which all the conditions found in existing larvas may have been 

 derived. This is shown in the following table : — 



Mr. Bury thinks it probable that in all groups (except perhaps 

 Holothurians), the radii of the abactinal part of the body (including 

 the regions of the right and left posterior enterocoels) bear a very 

 definite relation to the mouth, anus, and water-pore of the larva ; that, 

 in fact, these organs mark out an interradius, which, since it contains 

 both mouth and anus, might be called ventral, or, as it is anterior to the 

 system of radial plates, and contains, when present, the preoral lobes, 

 may be called anterior. The latter term seems preferable, but, if it is 

 used, we must be guided by the situation of the water-pore rather than 

 by the indefinite and variable positions of the mouth and anus, when 

 seeking for an anterior interradius in adult forms. 



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