SIPUNCULID^ 365 



pearance in front of the collar. Two pairs of rigid bristles 

 (Fig. 160) arise on the trunk, each of which belongs to an 

 ectodermal cell. A third pair is subsequently added to 

 these. Selexka is inclined to compare them to the setae of 

 the Annelida. The latter arise, it is true, as ectodermal 

 structures, though not in so simple a way as here. 



General Considerations. — With the limited knowledge 

 that we possess of the development of the different genera 

 of Sipunculidce, it is difficult to pass judgment on the syste- 

 matic position of this group. Until quite recently the 

 Sipunculidce, with the Echiuridce, were usually united into 

 the group of Gephyrea. The grounds which led to this 

 association were rather of an external nature. A comparison 

 of the anatomical and embryological data proves that the 

 two groups exhibit no special resemblances. The so-called 

 proboscis of the Echiuridce corresponds to the elongated 

 cephalic lobe of the larva; the mouth lies at its base, but 

 in the Sipunculidse at the tip of the proboscis. The cephalic 

 lobe entirely degenerates even in the larva. (Comp. Fig. 

 159, p. 362, and Fig. 145, p. 309.) The differences in 

 the structure of the nervous system, and especially the 

 musculature, which separate the Sipunculidce from the 

 Echiuridce and also from the Annelida, are striking. It 

 seems very doubtful whether these differences can be main- 

 tained after a comprehensive knowledge of the development 

 of the Sipunculidce, and if so, to what extent. The chief 

 point is whether or not the Sipunculidce are to be derived 

 from segmented forms, i.e., whether they are related to 

 the Annelida. In the Echiuridae we saw that a segmenta- 

 tion was indicated in the larva, and through this and the 

 remaining structural conditions of the larva we acquired 

 an insight into their relationships to the Annelid stem. In 

 Sipunculus such indications are lacking. To be sure, the 

 mesoderm here also splits into two layers, progressing from 

 in front backwards, and the differentiation of the nervous 

 system, which, however, is aberrant in being produced from 

 an unpaired fundament, takes place in the same direction ; 

 but no transitory segmentation is indicated, a head kidney is 

 not present, the preoral ciliated band is lacking, and the 



