HYDROZOA. 747 



Various views have been held as to the relation and nature of the Medusa. 

 That it is merely a specialised sexual organ, or that it is a colonial organism, no 

 person with a knowledge of its development and varieties of form, would maintain 

 at the present day. It is generally held to be a zooid, polymorphic with a Hydroid 

 and specialised for the purpose of reproduction, this function having become re- 

 stricted to certain individuals of a Hydroid colony which are detached, and have 

 acquired special facilities for locomotion in order to disseminate the race. Another 

 view, strongly advocated by W. K. Brooks, which appears to have much in its favour, 

 is to the effect that the primitive Hydrozoon was a free Hydroid 1 which gradually 

 acquired a more and more perfect organisation for swimming : that when it had 

 thus become a larval stage in an ontogeny, it began to multiply by gemmation : 

 that its progeny were at first all detached and became Medusae, but that at a later 

 stage of evolution some remained in connection with one another in the Hydroid 

 stage, whilst the development of the Medusae became accelerated in situ : that 

 finally the Medusa has in many instances become by acceleration of the develop- 

 ment of its sexual products a degenerate sessile organism. 



The points in favour of this view, as applied to the Craspedota, are, briefly put, 

 the following : (i) The direct development of the Trachy medusae, see pp. 752-3. 

 (2) The existence of a free sexual Hydroidean Hydra ; and of a free hydroid larva, 

 the Actinula, in two genera of Hydroidea, unless the latter is to be regarded as an 

 instance of precocious development. (3) The fact that the development of the 

 Medusa in the Hydroidea is abbreviated as shown (a) by the way in which the 

 bell and velum are formed not by simple growth but from an entocodon or ecto- 

 dermic thickening 2 a structure not seen in Trachymedusae^ and (&) by the fact that it 

 is so commonly degenerate (p. 762), and (<r) that it is very generally produced on the 

 hydrocephalis, a place where hydranths never bud, or on a blastostyle, a specialised 

 Hydroid. (4) The fact that among the Hydroidea closely related genera differ 

 greatly in the character of their reproductive zooids (p. 768), and that the develop- 

 ment of the sexual cells is accelerated in many instances (pp. 767-8). (5) That the 

 Medusa is not purely a reproductive organism but feeds, &c., like any other organism, 

 and that its specialisation is as Brooks says ' to enable it to live out its own life.' 

 (6) The fact that a Medusa may reproduce itself, but as a Medusa only, by budding. 

 What is true of the Craspedota appears to be true also of the Acraspeda. Pelagia 

 developes directly from the ovum \ others, so far as their development is known, by 

 strobilisation (p. 782), a process which has probably been attained by abbrevia- 

 tion, though it is perhaps not necessary to assume with Glaus that the primitive 

 Acrasped Hydroid was a colonial organism (cf. Untersuchungen iiber die Organisa- 

 tion, &c., der Medusen, p. 18). Gotte has recently advanced the view that the sub- 

 class in question should be separated from the Hydrozoa, and that it should be 

 united with the Anthozoa and Ctenophora in a division of Coelenterata to be termed 

 ' Scyphozoa.' But he appears to exaggerate the degree to which the ectoderm is 



1 Brooks says ' a solitary swimming hydra or actinula.' Why ' swimming ' is not clear. The 

 natatory Hydroid of the Trachymedusae is a larval form, but a free adult would more probably be 

 a creeping form, much like Hydra. Swimming, unless effected by cilia, requires a specialised organ 

 for the purpose. 



2 For an end odermic entocodon in Coryne pusilla seeWeismann, Die Entstehung der Sexualzellen 

 bei der Hydromedusen, p. 53, and for the way in which an entocodon may have originated, ibid, 

 pp. 259-60. 



