336 E. W. MAOBRIDE. 



an impossibility. A few Aunelid eggs have, however, very 

 little yolk, and the larvje commences a free life in the gastrula 

 stage.i Even here, however, if the blastosphere be compared 

 with that of an Echinoderm, one is struck at once by the com- 

 paratively small number of cells it has, and one understands 

 why the mesodermal rudiment should be represented by a 

 single cell. The small number of cells is doubtless due to the 

 comparatively large quantity of yolk, even if it be fairly 

 uniformly distributed. This is probably, however, not the 

 only reason why the coelomic gut pouch is not found in the 

 Annelid larva. If we compare Echinoderm larvae with one 

 another, we find that the blastocoele or segmentation cavity 

 and the coelom vary inversely with regard to one another. 

 Thus in the creeping larva of Asteriua the coelom is very 

 spacious, and the blastocoele reduced to a mere slit ; in the 

 pelagic larva of Asterias, on the other baud, the blastocoele is 

 exceedingly large, and the coelom has the form of two narrow 

 tubes the lumen of which is in parts occluded. A similar 

 comparison can be made between the ordinary Tornaria larva 

 of Balauoglossus and Bateson^s larva. The reason of this 

 difference is not far to seek. It is to the over-development of 

 the blastocoele with its contained jelly that pelagic larvse owe 

 that transparency which is so invaluable to them ; hence the 

 great development of the blastocoele in pelagic larvse and con- 

 sequent feeble development of the coelom. 



Now whatever may be the functions of the coelom in 

 Echinoderms, in Annelids its main functions are excretion, and 

 the production of the sexual cells. Of these the first is 

 performed in the Trochophore (the characteristic Annelid larva) 

 by the so-called protonephridium, and the second has, of 

 course, no place in larval economy. Hence, if we regard the 

 Trochophore as bearing somewhat the same relation to the 

 Echinoderm larva as the Zosea of the crab does to that of 

 Penseus, we see why the coelom should have been entirely sup- 

 pressed and the mesoderm represented only by a few large cells. 



1 Compare figures given iu Korschelt and Heider's ' Lehrbuch der Ver- 

 gleichende Embrjologie.' 



