98 MORPHOLOGY. 



I have myself traced the development of the egg of a Tijaropsis as far as the plaiuila stage ; 

 but though the planula; continued to live in my jars for some weeks, they ultimately perished 

 without passing into any further phase of their metamorphosis. 



The class of observations here enumerated enable us to complete the circle of hydroid 

 development, and justify us in the enunciation of a second proposition, which, taken along with 

 the former one, will express the entire life series of the hydroid : 



The ova of the medusiforu bud undergo, like those of the sporosac, a con- 

 tinuous DEVELOPMENT, BY WHICH THEY BECOME TRANSFORMED INTO HYDRIFORM TROPIIOSOMES, 

 WHILE THESE TROPHOSOMES ULTIMATELY GIVE ORIGIN, BY BUDS, TO MEDUS.E IDENTICAL WITH 

 THOSE FROM WHOSE OVA THE TROPIIOSOME WAS DIRECTLY DEVKLOPED. 



It will be further seen, from the facts now stated, that the earliest stage of the hydroid tro- 

 Ijhosome is always free and locomotive, and that it shows itself under one or other of two types. 

 One of these types is presented by the great majority of the Hydroida, and has been described 

 above as \hQpIanulaol Sir J. G. Dalyell ; the other occurs in the genus Tubularia, and apparently 

 also in Myrotltcla, Hydra, and Actinogonium, and has been already described under the name of 

 adinula. Every hydroid, if we except such forms as may be proved to pass to the medusal con- 

 dition directly from the egg, thus commences its free existence either as a planula or an actinula. 



Direct Development of the Medusa from the Eyg. — In by far the majority of cases in which 

 the development of the hydroid has been successfully traced, the life series of the individual has 

 presented a non-sexual hydra-like form, interposed between the ovum and the directly or indi- 

 rectly sexual medusal form. 



Against the absolute universality of this law, however, certain oi)servations have been adduced, 

 as tending to show that in some cases a direct development from the egg to the medusa takes 

 place without the intervention of a non-sexual trophosome. There is no reason why this should 

 not be so, and yet a careful examination of the cases adduced in support of it will render it 

 evident that most of them afford no evidence which can be relied on as conclusive in favour of the 

 direct development of the medusa from the egg.^ 



It is chiefly among the ^Eginidan and Geryonidan medusaj that cases believed to afford evidence 

 of direct development have been observed. The first observation bearing immediately on this 

 question is due to Johan. Miiller,' who captured, on several occasions in the sea, at jMarseilles and 

 Nice, a minute free-swimming hydroid. It was of an oval form, about half a line in its longer 

 diameter, ciliated over its entire surface, with two tentacle-like processes near one end, and 

 having at the opposite end an opening which led into a central cavity. 



Muller considers this little animal to have been developed directly from the egg, and from its 

 resemblance to a peculiar two-tentacled medusa which he obtained in considerable abundance at 

 Nice, he believes himself justified in regarding it as one of the stages in the development of this 



' Among the Discophoka a single case also of direct development has been made kuowu ; that, 

 namely, of Pelaijia, a medusa which has been sliown by Krohu to be developed from the egg without 

 tlie intervention of a Sciiphosloma or polypoid form. 



• -Muller's ' Archiv,' 1851, p. 272. 



