On (he Stnirfurt' of a Species of Millc|inra. 170 



to ei^lit ill iiuiiilKr. In liUli)I<)i,'ic;il slnictiin', as also in clifiiii- 

 cal <'oin|)ositi((ii •. the conilla of fix- Junius Millijxmi srciii to 

 sho« iio marked ditYtTi'iu'cM from Aiilhozoan coralla. 



The /(K)i(ls arc of two kinds, Tlu'oni', short and stont , oc(ii[)i»'s 

 tlu' lari^iT central caliclesof the systems, has fntm fonr to six short 

 kiiohU'd teiitacli's. and is provided with a mouth and certain ^'as- 

 tric cells, closely resendtlini^ (hose ti<;nred l>v Allman as occurring 

 in (rftinii'irui iiiiplf.rit +. 'I'he other kind occupies the smaller cali- 

 rles, is longer and mon- slender than the m nilhed zooid, has from 

 Hv«» to twenty tentacles, and tjo trace of a mouth. Tho usual num- 

 ber of tentacles in the mouthless zooid is about twj-lve to HftX'L'U. 

 The tvntacles are Iar<j;er than in the mouthed zooid, and disposed 

 Rt irrei^'ular interxals along the body. 'J'hey show the transverse 

 striation, or apparent septa, so characteristic of the tentacles of hy- 

 droids. 'I'hev have spheroidal heads comi)osed f)f masses of thread- 

 cells. 



'J'he zooids of both kinils are [)ro\ided with well-marked Icjugi- 

 tudinal muscular tibres, which are disposed in bundles, and are 

 attached inferiorly to th(^ vessels of the hydro[)hyton which join 

 the soniat ic cavi: y at the base of the zooids. Circular muscular fibres 

 are possibly also present. As in Hdlifmra. only a thin layer at, 

 the surface of the coral is living. 



The soft parts of the hydrophyton consist of a network of canals 

 and vessels occupying the corresponding canals in the corallum. 

 The canals are comj)osed of aji ectodenn and an endodonn. The 

 ectoderm rests on a thin layer of membrane. It is mainly composed 

 of fusiform finely granular cells with an oval nucleus, but is much 

 modified in certain regions. In the upper part of the living layer 

 its cells are abundantly converted into the parent cells of thread- 

 cells, and on the actual surface into a layer of prismatic cells 

 showing at the very surface hexagonal outlines. This layer is 

 believed to lx> continuous over the whole outer surface of the coral. 

 It is continued down into the calicular cavities, and in the con- 

 tracted condition almost closes their orifices. The endod'^rm con- 

 sists of two elements — yellow pigmented cells closely similar to 

 those of other hydroids, and small transparent highly n-fracting 

 globules. The i)igmented cells are abundant in the somatic cavi- 

 ties of the zooids, and in the canals and \('ss(>ls of the hydrophyton. 



Thev impart a bright yellow colour to the tips of the tubercles 

 of the living coral. The canal-system of the hydrophyton anasto- 

 moses most freely with the somatic cavities of the zooids, and 

 establishes a free communication between them. Two kinds of 

 thread-colls are present. The one is of the peculiar form occurring 

 only in llydrozoa, viz. that which has in the expanded condition a 

 short, wide, bladder-like structure at the base of the thread next 



• StriK'tiiri' iind Classification of Zoopbytos, h\- J. I). Djiiia (Fliiliidclnliia, 

 184fi), Apponflix. p. I.'W). CoralH and Coral hlnnds, fjuiul. (London, 1872), 

 p. 10.S. 



♦■ Gyinnohlaslic an<l Tiibwlannn H>droiil>4, |il. viii fip. .">. 



