Il8 MANUAL OF ZOOLOGY. 



cent towards the close of its existence, and the swimming-bell 

 becomes reversed or atrophied. Lastly, in Clavatella, the 

 sexual zooid, though free and locomotive, is not provided with 

 a swimming-bell, but creeps about by means of suctorial discs 

 developed on branches of the tentacles. 



As we have seen, the generative buds of the Corynida may 

 exist in the following chief forms : i. As " sporosacs," or 

 simple closed sacs, consisting of ectoderm and endoderm, with 

 a central cavity in which ova and spermatozoa are produced. 

 2. As " disguised medusoids," in which there is a central manu- 

 brial process and a rudimentary system of gonocalycine canals ; 

 but the gonocalyx remains closed. 3. As complete medusoids, 

 which have a central manubrium, a complete system of gono- 

 calycine canals, and an open gonocalyx ; but which never be- 

 come detached. 4. As perfect medusiform gonophores (fig. 

 42), which are detached, and lead an independent existence 

 for a time, until the generative elements are matured. In 

 whichever of these forms the gonophore may be present, the 

 place of its origin from the trophosome may vary in different 

 species of the order, i. They may arise from the sides of the 

 polypites, as in Coryne and Stauridia; 2. They may be produced 

 from the ccenosarc, as in Cordylophora ; 3. They may be pro- 

 duced upon certain special processes, which are termed " gono- 

 blastidia," as in Hydractinia and Dicoryne. These gonoblastidia 

 (fig. 43, g) are processes from the body-wall or coenosarc, which 

 closely resemble true polypites in form, but differ from them 

 in being usually devoid of a mouth, and in having shorter 

 tentacles. They are, in truth, atrophied or undeveloped 

 polypites. 



The gonoblastidia are the " blastostyles " of Prof. Allman, 

 and are usually columniform in shape. They may carry spo- 

 rosacs, or medusoid gonophores ; and they may be naked, or, 

 in other orders, they may be protected within a chitinous re- 

 ceptacle or "gonangium." 



As regards the development of the Corynida, the embryo is very gener- 

 ally, though not always, ciliated at first, when it is knoM'n as a " planula ; " 

 but in one form the embryo leaves the gonophore as a free and locomotive 

 polypite, and in another it is non-ciliated and amoeboid. The ''planula " 

 is a minute ciliated cylindrical body, which swims about actively in the 

 water. The embryonic cells of which it is composed clivide into an outer 

 and an inner layer, enclosing a central cavity, and it next passes into a 

 condition which is common to the embryos of the Ccelenterata generally, 

 and to which Haeckel has applied the name of "gastrula." At this stage, 

 it consists of an ovate or rounded body, with a single central cavity, which 

 communicates with the exterior by an aperture placed at one pole. The 

 wall of this central cavity consists of two layers, an outer and an inner, 

 corresponding with the ectoderm and endoderm of the adult, and also with 



