DEVELOPMENT. 



279 



ducts of these glands are five in number, and open to the exterior 

 through five openings in the skeletal plates (genital plates) around 

 the apical pole (tigs. 206, 222). In the irregular Spatangidce the 

 generative organ of the posterior interraclius is .always absent, and 

 the number of glands may be three or two. In the Asteridea the 

 five pairs of genital glands have the same interradial arrangement : 

 sometimes however, they project into the arms : the apertures for 

 the exit of the generative products lie on the dorsal side, and in 

 each interradius two places may be found, each of which is pierced in 

 a sieve-like manner by a number of such openings (fig. 223). In the 

 Ophiuridce ten lobed generative glands composed of a number of 

 blind tubes are developed around the stomach ; their products pass 

 through special passages into pouches, and from thence to the 

 exterior through paired slits on the ventral side between the arms. 

 The generative glands of the Crinoidea 

 are concealed in the arms and pinnules. 

 In the Holothurians, the generative 

 organs are reduced to one branched 

 gland, the duct of which opens to the 

 exterior not far from the anterior pole 

 of the body on the dorsal side (fig. 219). 



The development of the Eckinodermata 

 presents as a rule a complicated meta- 

 morphosis, and is characterised by the 

 possession of bilateral larval stages. 

 Many Holothurians are developed with- 

 out passing through the-e larval stages, 

 as also are certain Sea-urchins, as 

 Anochanus, Ilemiaster, and some Aste- 



roidea, which are either viviparous (Amphiura squamata) or lay only 

 a small number of eggs, and protect them during their development 

 in a brood pouch. In these cases also the first stage is a ciliated 

 embryo, which is either developed directly or passes through a much 

 simplified metamorphosis. 



In the cases of a complicated metamorphosis, the ovum, after under- 

 going a nearly equal segmentation, gives rise to a spherical embryo, 

 the cellular wall of which is ciliated and encloses a central gelatiiio is 

 substance (fig. 103). A pitlike depression of the cellular wall gives 

 rise to the first rudiment of the alimentary canal, and the opening 

 of this depression (gastrula mouth) to the anus. The ciliated embryo 

 becomes elongated and gradually takes the form of a long, oval, more 



FIG. 223. Part (if the inter-radius of 

 a star-fish (Sol outer) with the gen- 

 erative glauds (G) and the groups 

 of pores (sieve plates) on the dor- 

 Bal skin (after J. Miiller and Tros- 

 chel). 



