McCrady.] 188 [Dec. 3, 



Brachiolaria (as we know its history from the admirable research ot 

 Alexander Agassiz), and with such Acalephas as the Ctenophorae and 

 iEginidae. The so-called caudal appendages of Bucephalus and the 

 Cercariaa, including two stumpy lobes, or long tentacula, one on each 

 side of an intermediate lobular unpaired organ, I compare with the 

 anterior extremity of Brachiolaria, which in the early stages of that 

 larva is its posterior extremity. The unpaired lobe, especially, is 

 developed into a long flat tail in the next stage of growth, like the 

 anterior projection of Brachiolaria, and the long streamers of Bipin- 

 naria. The warty adhering organs on this anterior projection of 

 Brachiolaria (which is also trifid, like the caudal extremity of Cerca- 

 ria) are probably homologous with the suckers of such forms as Poly- 

 stoma, and are analogously used to anchor the larva while undergoing 

 resorption into the young star-fish. The digestive system of the 

 Trematodes would then require to be interpreted as in the condition 

 of that of Brachiolaria, before the mouth is formed, and when the 

 opening subsequently to be restricted to the functions of an anus, 

 fulfils also those of a mouth. Possibly the ventral sucker of the Cer- 

 caria may be a specialization of the depression on the ventral surface 

 of Brachiolaria preceding the formation of the mouth. But however 

 that"raay a be, this synthetic condition, in which a single opening fulfils 

 at once the functions of mouth and anus, is precisely that which we 

 see, not only in Turbellarians, but among all Acalephae; more espe- 

 cially for our] present purpose among Ctenophorae, which not only 

 bear their sense organs at the opposite pole, but in a large number of 

 forms present two remarkable tentacula, which I compare with those 

 of Bucephalus, and also (since they are lodged in special tubular 

 chambers) with the single peculiar proboscis of the Nemertians. 

 When the water-tubes are sprouting, as two horns, from the blind 

 digestive tube of Brachiolaria before the mouth is formed, we have a 

 condition of the digestive system nearly identical with the ordinary 

 biramic form among Trematodes; while again the isolation of the 

 water system from the digestive among Trematodes and Cestodes, is 

 merely an advance in specialization upon the synthesis of the two 

 systems in Ctenophorae ; we can still directly compare the one case 

 with the other, for the single or double contractile vesicle and open- 

 ing of the water system at a point opposite the mouth in Trematodes, 

 Cestodes and Nemertians, correspond precisely in formation and 

 position to the two contractile " coeliac apertures " of Ctenophorae, so 

 well figured and described by Agassiz, and which also have an excre- 



