IV PHYLUM CCELENTERATA 153 



begins within the maternal tissues, i.e. while the oosperm or im- 

 pregnated egg-cell is still contained in the gonad of the medusae or 

 in the sporosac. The oosperm divides into two cells, then into 

 four, eight, sixteen, &c. Fluid accumulates in the interior of the 

 embryo, resulting in the formation of a blastula or hollow globe 

 formed of a single layer of cells (Fig. Ill, A). The blastula 

 elongates, and the cells at one pole undergo division, the daughter- 

 cells passing into the cavity, which they gradually fill (B). At 

 this stage the embryo is called a planula : it consists of an outer 

 layer of cylindrical cells — the ectoderm^ — which acquire cilia, and an 

 inner mass of polyhedral cells — the endoderm. In some cases the 

 planula arises by a different process : a solid morula is formed, the 

 superficial cells of which become radially elongated and form 

 ectoderm, the central mass of cells becoming endoderm. By 

 means of its cilia the planula swims freely, and before long a 

 cavity appears in the middle of the solid mass of endoderm, the 

 cells of which then arrange themselves in a single layer around 

 the cavity or enteron (C, al). The planula then comes to rest, fixes 

 itself at one end to some suitable support, and becomes con- 

 verted into a simple polype or hydrula by the attached end 

 broadening into a disc and the opposite extremity forming a 

 manubrium and tentacles. The hydrula soon begins to send off 

 lateral buds, and so produces the branched colony. 



In Tubularia the oosperm develops, while still enclosed in 

 the sporosac, into a short hydrula, which, after leading a free 

 existence for a short time, fixes itself by its proximal end, buds, and 

 produces the colony. In Hydra development begins in the ovary, 

 and is complicated by the fact that the ectoderm of the morula 

 gives rise to a sort of protective shell : in this condition the 

 embryo is set free, and, after a period of rest, develops into the 

 adult form. 



Order 2. — Trachylin^ 



Greneral Structure. — The members of this order are all 

 medusae : no zoophyte stage is certainly known in any of them, and 

 several species have been proved to develop directly from the egg. 

 They thus differ from the members of the preceding order in the 

 fact that no alternation of generations ordinarily occurs in their 

 life-history. 



Most species are of small or moderate size, the largest not 

 exceeding 100 mm. (4 inches) in diameter. The gelatinous tissue 

 or mesoglcBa of the ex-umbrella is usually well developed, giving 

 the medusa a more solid appearance than the delicate jelly-fish of 

 the preceding order: this is well shown in Fig. 112, in which the 

 apical region of the umbrella has a comparatively immense thick- 

 ness. The tentacles are also stiff and strong, and are always solid 

 in the young condition, although they may be replaced in the 

 adult by hollow tentacles. 



