432 



THE ENTEROPNEUSTA. 



With these facts in mind, if we compare an echinoderm larva with that of 

 Balanoglossus, it will be seen that the resemblances between them in general form, 

 and in the arrangement of the ciliated bands, are not of special significance; or at 

 least they can be explained on the assumption that both echinoderms and entero- 

 pneusta are descended from arthropods, and that their larval forms are similarly 

 adapted to a temporary pelagic existence. 



The differences are more fundamental, for in the echinoderm the mouth lies 

 on the primitive neural surface and is still surrounded by the nerve cord, or a rem- 

 nant of it. Although the larval mouth of the echinoderms is said, in some cases, 

 to disappear, or to close for a time, it opens again, or a new one is formed very 

 close to where the old one was last seen. There are no indications that the per- 

 manent mouth of the echinoderms is a hasmostoma, and there are no indications 

 that a functionless remnant of a neurostoma is present. In the enteropneusta 

 the case is different. The permanent mouth lies on the haemal surface, and out- 

 side of the medullary plate; it cannot, therefore, be compared with the functional 

 mouth of the echinoderms, annelids, molluscs, or arthropods. Moreover, in 

 the enteropneusta there are definite indications of a primitive infolding of the 

 neural surface that perforates the medullary plate where the primitive mouth should 

 be located. This opening is not comparable with the hydropore, because it lies on 

 the neural side, not the heemal. It is, however, comparable in position and de- 

 velopment with the neurostoma of annelids and arthropods, and probably repre- 

 sents the subneural gland of the tunicates, and the infundibular tube of true verte- 

 brates. (Compare Figs. 43, 44, 284-297.) 



The Enteropneusta and the Arthropods. — The enteropneusta are prob- 

 ably descendants of primitive arthropods in which the essential features of the class 

 were definitely developed. Through one of those inexplicable internal condi- 

 tions that are exemplified in other sub-phyla of the acraniates, the enteropneusta 

 have failed to develop to their full extent many of the characteristic structures of 

 segmented animals. The power of growth is vigorous enough, for the dwarfed 

 forms of other phyla do not occur here, but the power of organic definition is 

 feeble, producing animals without conspicuous appendages, or outgrowths of any 

 kind, and without a sharp definition or specialization of the organs ordinarily 

 segmentally arranged, such as neuromeres, myotomes, sense organs and coelomic 

 chambers. We have frequently seen this condition in the abnormal embryos of 

 Limulus, where certain individuals appear, as it were, to have passed through a 

 flame that softened or partly melted the usual surface details (Fig. 184), or that 

 reduced a whole group of metameres to a single appendage, or to an ill-defined 

 unspecialized mass of cells. (Fig. 186.) A somewhat similar condition is familiar 

 enough in the maggot-like larvae of many insects and arachnids, and especially 

 in those forms where these conditions become more and more accentuated in the 

 later stages, as in Pentastoma, and in innumerable parasitic Crustacea. 



The initial factor in these cases is, no doubt, a defective germinal structure 

 that determines the character of the older stages and rigidly prescribes the mode 



