POSTLARVAL DEVELOPMENT. 119 



ANATOMY AND HISTOLOGY OF LARVA AND YOUNG POLYP. 



A number of free-swimming larvae were preserved in corrosive acetic, 

 and later studied by means of transverse and longitudinal sections, when all 

 were found to be at about tlie same stage of development. Only the impor- 

 tant features in which they differ from mature polyps will be here noticed. 



The larval ectoderm is somewhat broader than the same layer in the 

 adult polyp. In sections it measures about 0.04 mm. across while that of the 

 adult is 0.03 mm. The outer surface is strongly and uniformly ciliated, the 

 enlarged base of each cilium being well defined. Numerous clear and gran- 

 ular gland cells occur, and towards the margin a zone of small nematocysts 

 0.015 mm. in length. The layer further differs from that of the adult in 

 containing a few Zooxanthellae, mainly restricted to the oral extremity. 

 These have been already noticed among the external characters as giving a 

 brownish color to the oral pole of the larva, and are apparently liberated 

 from time to time. 



The larval ectoderm is broader at the base, where it measures 0.06 mm. 

 across, and, in addition, has undergone certain histological modifications. 

 The cells as a whole seem more compactly arranged, gland cells are less 

 numerous, and a nerve layer is present ; but nematocysts do not seem more 

 numerous than elsewhere. The whole structure recalls the aboral sense 

 organ which has already been found by McMurrich, Appellof (1900), and 

 myself (1902, p. 524), to occur in certain actinian and coral larvae, and is 

 evidently widely, though not universally, present in these two groups. The 

 characteristics of the organ, however, are usually more conspicuous than in 

 the present species, especially as regards the degree of development of the 

 nervous elements. 



The stomodseal communication between the exterior and the larval cavity 

 was already established in all the larvae studied. In most the lumen is circu- 

 lar, while in others it is slightly oval. The surface is more strongly ciliated 

 than that of the outer ectoderm, and fewer gland cells occur; the two agree 

 in the presence of Zooxanthellse, though these are never found in the ecto- 

 derm of the adult stomodseum. 



The mesenterial filaments on pairs i and n on plate 8, fig. 52, stand 

 out conspicuously from the endoderm on account of the number and deeply- 

 staining character of their nuclei, and their histological structure recalls that 

 of the stomodaeal ectoderm. In the larvae, however, the organs are not yet 

 rounded off from the mesenterial endoderm. 



