INTRODUCTORY OBSERVATIONS, 95 



ol)sei*ved Iby Sars and Daly ell, where the ova were developed into 

 a Hydroid Zoophyte, propagating itself in that condition by 

 gemmation, and afterwards re-assuming the Medusoid form by 

 fissiparoiis division, there would, perhaps, be little hesitation in 

 pronouncing the Medusa to be the normal form of the species. 

 In some zoophytes, however, the case is different. Observation 

 has shown that the natatory progeny of Tuhularia are the larval 

 state of that genus, which, after swimming about for a short time 

 in freedom, affix themselves to other bodies, and are developed 

 into zoophytes of the parental type. Mr. W. P. Cocks has seen 

 the same thing in the curious genus Myriothela ; and though the 

 young of these genera do not assume the perfect medusoid form, 

 as in the allied genus Cort/ne, we can scarcely doubt that both 

 partake of the same general character, and must come into the 

 same category. The extreme delicacy and minuteness of the 

 medusoids of the latter genus, as well as those of the Campanu- 

 lariadce^ have baffled the attempts of naturalists to trace them 

 beyond an early stage, but in most cases they appear to have a 

 very transitory existence. By some they are considered to be 

 the perfect and adult state of the animal, while others take them 

 to be merely partially developed sexual organs, endowed with a 

 locomotive power the more readily to diffuse the ova in distant 

 localities. The close affinity of these genera to the Tuhulariadce 

 and Sertulariadce (the embryology of both of which is known)^ 

 would lead to the belief that the zoophyte will be found to be 

 the typical form in all these families. 



In the systems of most modern authors, the Pohjzoa are re- 

 moved from the Zoophyta, and take their rank in the sub-king- 

 dom Mollusca. Their relationship with this group is established 

 through the Tumcata, a kind of pseudo-molluscans, with the 

 lower or compound forms of which they have a close affinity. 

 Their dissimilarity to the typical Mollusca, however, is very 

 striking, and I think there are several reasons that might justify 

 us in retaining them among zoophytes, where they still hold a 

 place in nearly all popular works on the subject. In everything 

 that regards external form, they are truly zoophytic. Their 

 branched and plant-like structure; their polypides issuing from 



VOL. III. PT. II. M 



