214 



LOWER INVERTEBRATES. 



in its undifferentiated form along both sides of the body and around tlie tail as a con- 

 tinuous flap. The posterior portion of the body is divided off from the anterior by a 

 transverse septum ; the anterior compartment is much the longer, and contains the 

 simple, straight intestine, which terminates just behind the septum and 

 the ovaries; the smaller caudal conij>artnient contains the male geni- 

 talia. 



These worms are probably related to the nematodes or thread-worms, 

 but their anatomical singularities are so numerous that it is well to keep 

 them apart. They were first described about the middle of the last cen- 

 tury by a Dutch author, Martin Slabber; for nearly one hundred years 

 little was added to our knowledge of these worms, but, during the last 

 forty years, tlieir structure and development have been quite thoroughly 

 worked up, principally by German zoologists. But even the complete 

 information which we now possess has not yet enabled us to form any 

 satisfactory or definite opinion as to the affinities of the group. 



The number of species known is only thirteen, which have been 

 separated into two genera, iSai/itta and Spadella. Some of the species, 

 at least, are cosmopolitan. The spawn may be obtained by confining' 

 the pregnant animals at the proper season, in most cases the spring or 

 early summer. The spawn drops to the bottom of the vessel and lies 

 there unattached. The ova are enveloped in a gelatinous mass, which 

 does not surround each ovum separately, but belongs to the whole mass 

 of eggs in common. In the species studied by Gegenbaur the process 

 of segmentation occupies some seven to nine days, and it is probable 

 that maturity is reached within a year. A singular interest attaches to 

 the literature upon the development of Sagitta, in that it includes a pub- 

 lication by Charles Darwin, which contains one of the few serious blun- 

 ders committed by the great English naturalist. He observed certain 

 fish-eggs which he mistook for, and described in 1844 as, the eggs of this 

 '^'/"'w M~/?''" worm, and it was not until many years later that the eiTor was corrected. 

 There is no metamorphosis, but the embryo gradually assumes the adult 

 form. The animals are predaceous, feed- 6 



ing upon small Crustacea and other pelagic 

 animals. 



We ought to mention here, rather than 

 elsewhere, two other aberrant types, which 

 have as yet been only very incompletely 

 studied. The little information we possess 

 indicates that they are related to Sagitta, 

 and also to the Nematodes. The first type 

 is known under the name of the Ch.<etosom- 

 iD^, and is probably the connecting link 

 between the Chaetognathi and thread-worms 

 proper. There are two genera, Mhabdo- 

 gaster and Chcetosoma, both marine, and 

 found crawling about over the surface of algffi. The former genus has been described 

 by Metschinkoff. The mature female measures only 0.36 millimeters. The anterior 



Fig. 203. - 



- Rhahtlogaster ci/iioides : a, immature 

 male; b, matui'e female. 



