iv VERMES ONTOGENY OF THE WORMS 111 



develops only at a later stage. Anteriorly, above the mouth and directly under 

 the ectoderm from which it is derived, lies a mass of cells the rudiment of the 

 brain. In many other worms with direct development the mid-gut is solid and 

 not hollow, in consequence of the large masses of yolk contained in the endoderm 

 cells (primitive macromeres). 



Since embryonic head nephridia have been observed in some OligocJiceta, the 

 chief differences between them and the Polychcetan Trochophora are the following : 

 Cilia are wanting in the former, and especially a pre-oral ciliated ring. Sensory 

 organs (eyes, ciliated tufts) are wanting. A body cavity is wanting, and also a proc- 

 todreum. On the other hand, as we shall see later, the mesoderm is much more 

 developed than in the Polychcetan Trochopkora above described. The parts of the 

 body present in the embryo represent the rudiments of the definitive organs, they 

 do not, as in a free-swimming larva, fulfil specific functions, and therefore seldom 

 disappear in the course of further development. The Gnathobdellidce form an ex- 

 ception, for in them a metamorphosis takes place, and, according to recent research, 

 the larval integument with its muscular and nervous systems, and further the 

 provisional trunk nephridia and the oesophagus, are said to disappear. 



Development of the Outer Integument. We may say, generally, that the outer 

 integument or body epithelium of the worms is derived from the embryonic or larval 

 ectoderm. The ectoderm either changes, without suffering any loss worth mentioning, 

 into the body epithelium ; or some parts of the larval ectoderm which perform 

 specific functions are thrown off ; or else certain portions of the larval ectoderm sever 

 themselves, and unite on all sides of the body within the larval ectoderm to form 

 the permanent ectoderm or body integument (e.g. Sipunculidce, Nemertinct}. We 

 consider these processes as a kind of ecdysis. In the Gnathobdellidce (Hirudinca} 

 alone it is said that the secondary ectoderm is not formed from the larval 

 ectoderm, and that the latter is quite lost. The larval and definitive sensory 

 organs, the larval and definitive nervous systems, and the larval and definitive seti- 

 parous sacs (of the Chcetopoda) are products of differentiation of the ectoderm. The 

 last mentioned are groups of glandular hypodermis cells, which sink under the 

 integument, the setse arising in them as secretions. The ectodermal setiparous glands 

 are enveloped by mesodermal elements which supply their musculature. 



Development of the Mesoderm and the Mesodermal Organs. The mesoderm is 

 for us a topographical conception. All that lies in the adult animal between the 

 outer integument and the intestinal epithelium belongs to the mesoderm. The 

 relations of the various mesodermal organs and systems of organs to the outer 

 integument are very varied. These relations are closest in the nervous system in 

 consequence of its dependence on the (ectodermal) sensory organs. As already 

 described (cf. p. 223), in many worms the central nervous system remains in the 

 integument, even in the adult animal, and is thus not mesodermal. The mesodermal 

 position of the nervous system is, however, the general rule, as it is even as early as 

 in the Platodes. As, however, it nearly always develops entirely separate from the 

 rest of the mesoderm, we shall describe its development first. 



The Brain. In some cases the brain, or a part of it at any rate, develops out 

 of the ectodermal neural plate (many Annelids, Sipunculidce}. The elements of 

 the neural plate probably arise in connection with the provisional or definitive sensory 

 organs of the head (eyes, neural tuft, tentacles), though this cannot be established in 

 detail. There are thus different parts which unite together to constitute it a sensory 

 nervous centre. The neural plate must thus represent an organ similar to the sensory 

 body of the Ctenophora or the marginal centres of the Medusce. It is often thrown 

 off with the larval integument (e.g. in Phoronis, Pilidium), and the oral nerve rino- 

 or the brain arises anew out of the secondary ectoderm. 



