Hyatt] 118 [March 5 



During this process the blastopore is carried inwards and the inter- 

 nal opening of the actinostome thus becomes the homologue of 

 the primitive blastopore of the hydroplanula, and also represents 

 the external orifice of the body in the Hydrozoa. This gullet- 

 larval, or gulinula stage, as we propose to call it, is additional to 

 the tentacular stage, which has in Hydrozoa been called the acti- 

 nula, — but occurs in point of time and succession in the Actinozoa 

 between the earlier gastrula or planula, and the later tentaculated 

 stage. This succession indicates very clearly that the tentacu- 

 lated stage of Actinozoa is not homogenous with the actinula of 

 the Hydrozoa, as indeed has been probable ever since the dis.cov. 

 ery of Protohydra and the noticeable correlation of this form and 

 the adolescent, larva of the marine Hydroida. Both agree in 

 being hydras without tentacles and almost conclusively show 

 that the primitive ancestor must have been similar to Protohydra. 

 Thus we have to consider the hydra stage as greatly abbreviated 

 in the Actinozoa, or not represented at all in many forms, and all 

 later stages than the planula are probably homoplastic and not 

 homogenous in their relation to the Hydrozoa. The Actinozoa 

 and Hydrozoa, in other words, differ in the succession of their 

 stages of growth, and there is an interpolation of new stages in 

 the Actinozoa which can be accounted for only on the supposition 

 that in these types the tentacles, the actinostome, and the mesen- 

 terial filaments are not inherited from the simpler Hydrozoa, but 

 originated independently in the Actinozoa. The pores, also, 

 which arise late in life in Actinozoa and are not found at all in 

 many forms of Hydrozoa, must have been independent in origin 

 like the actinostome. 



This actinostome, though an invagination in the embryos of 

 existing Actinozoa, was evidently not subject to invagination in 

 primitive forms. The mode of formation of the Actinostome by 

 invagination may therefore, in accord with our theory, be sup- 

 posed to be secondary and to have arisen through peripheral out- 

 growth of the parts around the mouth. This would have caused 

 the actinostome first to assume the form of an inverted funnel 

 lined by the ectoderm, and finally have led to the introduction of 

 invagination. The formation of an invagination homologous 

 with an actinostome occurs in Cassiopea,but very late in the life of 

 the larva after it becomes attached (Kowalevsky, Soc. Friends 



