THE CCELENTERATA 161 



of food, and so is spoken of as the nutritive zooid, but two other kinds 

 of person are to be found. In the first place sexual reproduction is 

 brought about by a special set of individuals in the form of small 

 saucer-shaped zooids produced as buds by the colony, but afterwards 

 cut off as minute free-swimming jelly-fish or medusae, and these are 

 the reproductive individuals. Secondly, these reproductive forms are 

 not budded from any part of the colony indiscriminately, but only 

 from specially modified hydroids, the blastostyles, which have no 

 mouth or tentacles, and consist of a long slender body. Here, then, 

 as a result of the division of labour among its constituent persons, we 

 encounter the phenomenon of polymorphism, or the production of 

 different forms of individual in one and the same species. It is even 

 more marked in the case of some of the allies of Obelia, and is met 

 with in various groups of the animal kingdom, as, for example, in the 

 social insects, ants and bees. The particular type of polymorphism 

 in the present case, since it involves three varieties of individual, the 

 hydranth, the blastostyle and the medusa, may be called trimorphism. 



The blastostyles are always to be found in a constant position. 

 Usually they are absent from the upper or younger end of the 

 hydrocaulus, but almost always present lower down. They occur 

 in the angle between the polyp stalk and the main stem (in the axil, 

 as we should say, of leaves) toward the upper side. Each is enclosed 

 in a special urn-shaped investment of the perisarc, the gonoiheca, 

 which is almost sessile and has not a distinct stalk like that of the 

 hydrotheca. 



The structural plan and histology of the whole colony is 

 very similar to that of Hydra, but perhaps, on the whole, more 

 simple. The ccenosarc of the hydrocaulus and hydrorhiza consists 

 of a series of tubes which are only loosely connected with the 

 perisarc save at the growing ends of the branches. It is composed 

 of an outer clear layer of ectoderm which lacks cnidoblasts, and an 

 inner more granular layer of entoderm whose cells are sometimes 

 ciliated, and bear flagella in order to assist in the circulation of the 

 nutritive fluid. The two layers are separated by the mesoglea, and 

 in neither are the muscular processes at all well developed. 



The nutritive zooid somewhat resembles Hydra, but has a circlet 

 of about thirty well-marked filiform tentacles. They are fairly 

 contractile and plentifully supplied with cnidoblasts. Their 

 ectoderm is on the whole similar to that in Hydra, but the entoderm, 

 which completely fills them, is composed of a single axial line of 

 peculiar cells. These cells are large and very vacuolated with a 

 nucleus near their centre, and are tough and elastic. The hypostome 

 is enormously developed, forming a dome-like enlargement between 

 the bases of the tentacles. Within, it forms a sort of initial gastral 



