478 CHARACTERS OF INVERTEBRATE ANIMALS 



limited to the surface-layer, while the deeper part of the mass 

 is cut off by partitions and merely consists of dead material. 

 In a living specimen of Tubipora the upper end of each tube is 

 occupied by a polype of similar character to those found in 

 Alcyonium, and adjacent members of the colony are connected 

 by living matter which extends along the uppermost platforms. 

 Microscopic examination of the skeleton shows that it is formed 

 by the intimate union of innumerable minute spicules comparable 

 in nature though not in shape to those of Dead- Man's Fingers. 



Other examples are Red Coral (Corallium rubrum) (fig. 2), 

 where there is a compact branching skeleton (composed of united 

 spicules) covered by the common flesh (ccenosarc) of the colony 

 from which the polypes project at intervals; Sea- Pens (Pennatula, 

 &c.), where the colony is feather - shaped with a series of indi- 

 viduals on each side, while the axis is supported by a firm rod; 

 and the Sea-Mats (Gorgonia, &c.), where the flat upright colony 

 is variously branched and supported by a horny skeleton, which 

 is covered by common flesh with polypes much as in the Red 

 Coral. 



CLASS II. HYDROIDS (HYDROZOA) 



The Fresh- water Polype, Hydra, has already been described 

 as a simple type of this class (pp. 465-471), which is pre- 

 dominatingly marine. The apparent simplicity of Hydra, it 

 should be noted, is in all probability the result of reduction, and 

 a much better idea of the group is to be obtained by briefly 

 considering one of those marine forms to which at different 

 stages of the life - history the terms "hydroid zoophyte" and 

 "medusa" are applied. The horny skeletons of zoophytes of 

 this sort are cast up on the shore in abundance by storms, and 

 some may be found growing near low-tide mark in rock-pools. 

 They are often confused with sea-weeds in amateur collections, 

 in which their branching skeletons are frequently to be found. A 

 very common British genus is Obelia (fig. 293), for which there 

 is no popular name. It will be remembered that in dealing with 

 Hydra it was pointed out (p. 472) that during summer, when 

 that animal is actively budding, two or three generations of indi- 

 viduals may be temporarily connected together. If these buds 

 were to remain united, and the budding process were carried 



