ANNELIDA 273 



P 



^^J uniform coat of cilia, and which at the most possess at the 

 ^^K anterior end of the body a tuft of cilia, which serves for 

 ^B steering (Atrochce, Fig. 122 A, B). 



^^B CLAPABfeDE UND Metschnikoff describe atrochal larvsB of Lumbnco- 

 nereis (?), and Vejdovsky those of Sternaspis.^ Both larvse are at first 

 spherical, but later become elongated (Fig. 122 B). The former possess 

 eye-spots ; the latter do not. A differentiation in the ciliation appears 

 even in these larvae, for in Lumhriconereis narrow zones, one toward the 

 anterior and one toward the posterior end, remain free from cilia, and in 

 Sternaspis the entire posterior end is without cilia (Fig. 122 A, B). 

 Inside one recognizes in the first form the sac-like fundament of the 

 digestive tract, in the latter, on the contrary, only a compact mass formed 

 of large entoderm cells. 



The further development of Lumhriconereis is marked by the ap- 

 pearance of setae in pairs at the posterior end of the body, thus giving 

 expression to the segmentation. At the same time the degeneration of 

 the cilia begins. In Sternaspis the entire ciliation of the body dis- 

 appears, and the larva continues to live in this naked condition for 

 some time, the segmentation of the body being as yet unrecognizable 

 (Vejdovsky, Eietsch). Its further development was not followed in 

 detail. 



It is difficult to say whether in the evenly ciliated larvae we have to do 

 with phylogenetically older stages than those represented by the Trocho- 

 phore. The incomplete development of the intestinal canal, especially 

 in the larva of Sternaspis, and also the subsequent development of this 

 worm, make it appear as more probably a derived form. Although in 

 Lumhriconereis the cilia in later stages are arranged into an anterior 

 and posterior region, this distribution is altogether too indistinct to be 

 referred to the anterior and posterior ciliated bands of the Trochophore. 



Although we are not justified in looking upon these 

 atrochal larvae as primitive forms, still it appears to follow 

 from the development of another Annelid that the larvae 

 which are provided with ciliated bands represent a stage 

 succeeding the atrochal forms. In Terehella Mechelii, which 

 was studied by Milne-Edwards, Clapar^de und Metschni- 

 koff, and later by Salensky, there arises from a larva, which 



* Sternaspis has been classed with the Echiuridae ; nevertheless in this 

 form, which is provided with setae, such a distinct segmentation is ex- 

 pressed, both externally on the body and internally, in the matter of the 

 arrangement of the muscles and blood-vessels (Kietsch), that this group 

 of Annelids — very aberrant, it is true— must still be placed among the 

 Chaetopoda. 



K. H. E. T 



