The porifera and coelentera . ription of the Hydroid and of the Medusoid.—In no group of the animal kingdom is polymorphism carried to agreater extent than in the Hydromedusae, yet, upon morphologicalanalysis, the numerous forms which individuals exhibit arcapparently all referable to modifications of one or other of twomain types—the Hydroid and the Medusoid. The Hydroid (hydriform jierson, hydranth, trophozooid) isrepresented in a simjile form by the genus Hydra, from which itderives its name. This presents (Figs. 3, 4, B and C) a tubular THE HYDROMEDUSAE body consisting of ectoderm, mesoglo
The porifera and coelentera . ription of the Hydroid and of the Medusoid.—In no group of the animal kingdom is polymorphism carried to agreater extent than in the Hydromedusae, yet, upon morphologicalanalysis, the numerous forms which individuals exhibit arcapparently all referable to modifications of one or other of twomain types—the Hydroid and the Medusoid. The Hydroid (hydriform jierson, hydranth, trophozooid) isrepresented in a simjile form by the genus Hydra, from which itderives its name. This presents (Figs. 3, 4, B and C) a tubular THE HYDROMEDUSAE body consisting of ectoderm, mesogloea, and endoderm, at oneend of which is a mouth, situated on a slight eminence (thehypostoTne); through the mouth the internal cavity (coelenteron)communicates with the outer world. Round the mouth are placedtentacles, which are hollow outgrowths of the body, their cavitybeing part of the coelenteron. In the hydroid thus composed the elements of the originaldiblastula are not far to seek : the primary two layers, ectoderm. Fifi. 3. Fig. 4. ?,.—}!jldm riridh, attacherl to a piece of weed, o!, ovary ; te, testis. 4. —Diagram exhibiting the i)lan of structure of liydroids. A, hydroid with widedisc, inanubriuni, and solid tentacles (Tubularian); Ji, hydroid with narrow disc andhollow tentacles (Hydra); C, transverse section of the body of a hydroid. All the figuresshow from without inwards ectoderm (strongly hatched), mesogloea (a thick black line), andendodenn (lightly hatched), surrounding the coelenteron. and endoderm, and the coelenteron, are still represented. Thesecretion of a mesogloea, the perforation of a mouth, and the out-growth of tentacles, are the main morphological differences betweenembryo and adult hydroid. Hydroids are either solitary or colonial. The solitary forms,such as Hydra, are capable of reproduction by a process of budding(Braem, 15 ; Seeliger, 16), (Fig. 4, B), in which a part of the bodywall, enclosing coelenteric cavity, protrudes laterally; th
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