2l8 ZOOLOGY 



external characters. The sexual organs, ovaries or testes, are 

 lobed bodies occurring usually in pairs in an interradial position. 

 These open by pores also interradial, and usually dorsal (Fig. 

 99, r. p.). There are typically five pairs of genital glands, but 

 in the holothurians the number is reduced to one. Fertilization 

 takes place outside the body, and usually the development is 

 wholly independent of the parent. In some instances however 

 the parent has special pouches in which development proceeds. 



254. Development. The fertilized ovum undergoes total 

 and practically equal segmentation, producing a ciliated blastula. 

 Gastrulation occurs by invagination resulting in ectoderm and 

 entoderm. The mesoderm is formed in two ways: (i) by mi- 

 grating cells budded from the entoderm into the segmentation 

 cavity (mesenchyma; Fig. 14, c}\ and (2) by the outgrowth of 

 ccelomic vesicles or pouches from the wall of the archenteron 

 or entoderm (true mesoderm). These latter outpockets of the 

 wall of the gut are those which give rise to the ccelom and to 

 the water vascular system (see 247). 



In the later larval development the cilia of the gastrula be- 

 come limited to two zones, a preoral and a preanal, and the 

 shape of the larva is much modified. Numerous paired, lateral 

 outgrowths serve to accentuate the fundamental bilateral sym- 

 metry. In most members of the group a marked metamorphosis 

 occurs in the passage from the larval to the adult condition. 

 During this change, the water vascular system and the mid-gut 

 of the larva are retained with the necessary modifications. 

 About these as a centre, what we might almost call a new animal, 

 the radiate starfish, begins to grow at the expense of the larval 

 organs which are absorbed by the amoeboid cells, and thus new 

 organs appropriate to the adult are formed. During this process 

 the bilateral symmetry of the embryo gives place to the radial 

 symmetry of the adult. While there is no reproduction by 

 budding there is a striking power of renewal of arms or other 

 portions which may be lost by injury, or in some instances by 

 self -mutilation. Arms are readily reproduced if the disc is 

 uninjured (stars, brittle-stars, and crinoids); portions of the 

 internal organs, as the digestive tract, are said to be regenerated 



