268 COMPARATIVE ANATOMY CHAP. 



former are prominences on the wall of the ordinary individuals 

 (zooecia), each of which carries a long seta, by means of whose free 

 movement particles of food are brought within reach, and the water 

 surrounding the colony is kept in motion. The stalked avicularia 

 are catching apparati which hold small animals fast till they die. The 

 mechanism is similar to that of a crab's forceps or a bird's beak ; a 

 movable mandible is hinged upon an immovable beak, and is 

 worked by special muscles, so that the forceps can open and shut. 

 Avicularia and vibracularia are, like the above-mentioned ooecia and 

 ovicells, regarded as metamorphosed individuals without intestine. 

 This assumed polymorphism of Chilostoman stocks is, however, as yet 

 weakly supported. In marine Bryozoa all the individuals of a stock 

 are connected together by a network of nerve threads, forming what 

 has been called a colonial nervous system. 



XVI. Ontogeny of the Worms. 



The eggs of worms are either holoblastic alecitlial, or holoblastic telolecithal. 

 In the latter case the yolk may vary very much in quantity. The course of seg- 

 mentation and gastrulation varies in correspondence with the constitution of the egg. 

 There is sometimes (in the holoblastic alecitlial egg, example Sagitta) a total and 

 tolerably equal furrowing, forming a cceloblastuia, and then by invagination a ccelo- 

 gastrula. Sometimes the furrowing is more or less unequal, and often connected with 

 the formation of micromeres (cf. p. 124, segmentation of Bonellia). We find, always 

 according to the quantity of yolk stored in the egg, all the stages from a coeloblastula 

 to a sterroblastula, from an invagination to an epibole, and from a coelogastrula to a 

 sterrogastrula. Alecithal eggs or telolecithal eggs with little nutritive yolk, ccelo- 

 blastulae and coelogastrulae, are found in those groups of worms in which a free- 

 swimming and independently feeding larva develops very early. This is the case in 

 very many marine worms, especially in Neinertina, Polychceta, Sipunculidce, Bryozoa, 

 Phoronis, Brachiopoda, and Chcetognatha. Telolecithal eggs with much nutritive yolk, 

 sterroblastulse and sterrogastrulse, are found in those cases where the developing 

 animal only begins to move and to feed very late, i.e. in an almost adult condition, 

 and especially where a so-called direct embryonic development takes place, e.g. 

 Oligochceta, Hirudinea, Eotatoria. 



Before passing on to describe the more important larval forms among the Vermes, 

 we will briefly describe the development of Eupomatus uncinatus (Serpulidae) (Fig. 178, 

 cf. also Fig. 92, p. 123). The blastula has a small blastoccel. The cells of the upper 

 (animal) half are smaller and more numerous than those of the lower (vegetative) half. 

 The former form the ectoderm, the latter the endoderm. At an early stage, at one side 

 of the blastula, which we may call the anal side, 2 round cells appear with remarkable 

 distinctness at the limit between the ectoderm and the endoderm. By the rise of 

 these primitive mesoderm cells the blastula becomes bilaterally symmetrical. 

 Besides an aboral or animal side and an oral or vegetative side, we can also distinguish 

 an anterior and posterior (where the 2 primitive mesoderm cells lie), and likewise 

 a right and a left, and a median plane. The two primitive mesoderm cells lie to the 

 right and left of the median plane. The vegetative or endodermal wall of the 

 blastula then becomes invaginated into the segmentation cavity to form the arch- 

 enteron while at the same time the ectoderm grows out over the invaginated part. 

 The process thus stands half way between invagination and epibole. The primi- 

 tive mesoderm cells, lying at the posterior edge of the blastopore, sink between 



