756 THE ANIMAL KINGDOM. 



forms an incrusting felt-work of tubes, the chitinoid perisarc of which is 

 thickened in the intervals, that of adjoining tubes fusing 1 . So too in the 

 Hydrocorallina, where the chitinoid perisarc is replaced by a coenosteum 

 or strong skeleton of Lime carbonate, and the hydrorhiza constitutes in- 

 crusting or more usually erect lamellate or columnar, &c., branching struc- 

 tures, simulating certain corals properly so-called. Pores or depressions in 

 this calcareous structure, either scattered or aggregated into systems and 

 sometimes furnished with raised walls, give shelter to the dactylozooids 

 and gastrozooids (infra, p. 758) of the colony : and in the genus Millepora, 

 newly grown strata of tubes are cut off from older and deeper strata by 

 calcareous platforms or tabulae. Similar tabulae cross the pores in the 

 same genus, and in two genera of Stylasteridae. 



Hydra is destitute of any natural or adventitious protective coat : 

 Microhydra invests itself with a coating of mud, &c., held together by a 

 secretion : Protohydra and Corymorpha nutans have a delicate cuticula. In 

 other instances a perisarc is present, thickish, brownish in colour, and com- 

 posed of lamellae ; but in Tiarella its outer portion is gelatinous. It is 

 secreted by the cells of the ectoderm, and is resorbed by them in places 

 where budding is taking place. It usually commences in the Tubularian 

 hydroids as a very delicate layer upon the hydrocephalis aborally to the 

 tentacles, is thicker on the hydrocope, and thickest on the hydrocaulus and 

 hydrorhiza 2 . It is confined to the two last named in the Campanularia- 

 like hydroids of Eutima, Octorchis and Aequorea ; but in most other Cam- 

 panularian hydroids there are perisarcal hydrothecae for the hydranths, 

 and gonothecae (or gonangia) for the blastostyles. The edge of a hydro- 

 theca may be thin, and thrown into plaits when the animal is retracted, 

 or cut into moveable or fixed teeth. In some Sertularidae it is furnished 

 with a valve-like operculum. 



The hydranths are usually very small in size ; the largest known, a 

 Monocaulus> does not exceed i J in. in length. When they are attached to 

 a branched stem, they are disposed either terminally or laterally on the 

 branches ; and in the latter case they may be pedunculate as in Tubulariae 

 and Campanularidae, or sessile and are then arranged either in a single 

 row as in Plumularidae^ or in a double row opposite or alternate one to 



reticulate hydrorhiza from which they spring ; see Allman, Gymnoblastic Hydroids, pp. 54-5, and 

 notes with Fig. 27. 



1 A living Hydractinia with calcareous skeleton has been described by Carter from West Africa 

 (A. N. H. (4), xix. p. 50; ibid. (5), i. p. 300). Calcareous species from the Chalk and Pliocene, a 

 pseudomorphic siliceous species from the Greensand are also known, together with extinct allied (?) 

 genera : see Carter, op. cit. ; Steinmann, yon Meyer's Palaeontographica, xxv. 1878. 



2 Perigonimus Cidaritis (Weismann, Entstehung der Sexualzellen, &c. p. 116) is invested by 

 a coat of mud, even extending on to the tentacles ; P. palliatus has a gelatinous coat (Allman, op. cit. 

 P- 3 2 5); P- vestitus a perisarc roughened by grains of sand (Allman, op. cit. p. 326). In the 

 Tubularian Bimeria the perisarc covers the hydranth and the bases of the tentacles. 



