88 MOLLUSCOIDA. 



(the introvert) is withdrawn, closes the aperture of the invagination 

 thus formed (Vol. i., Fig. 159, rm, p. 362). 



It appears from the above, that the Sipunculidae cannot be brought 

 into any near relationship to the Molluscoida. Since, after careful 

 consideration of the facts of ontogeny, we agree with HATSCHEK 

 (No. 6) in doubting the near relationship of the Molluscoida and 

 the Entoprocta, we are unable to adopt the view of those zoologists 

 who use the structure and development of Pedicellina for explaining 

 the conditions of the Molluscoida. This latter view has recently 

 been adopted by BARROIS, SEELIGER, DAVENPORT, and EHLERS. 

 Among these, EHLERS approaches our standpoint, in so far as he 

 also denies that the crown of tentacles in the Entoprocta is homo- 

 logous with that in the Ectoprocta. For the reasons which compel 

 us to exclude the Entoprocta from the Molluscoida, we must refer 

 the reader to the following chapter (p. 101).* 



The remarkable genera Rhabdopleura and Cephalodiscus have also repeatedly 

 been brought into near relationship to the Ectoproctous Bryozoa and Phoronis. 

 Meantime, the researches of HARMER, which have recently been confirmed by 

 SPENGEL (No. 13),f and EHLERS (No. 3) have revealed a striking similarity 

 of organisation between Cephalodiscus and Balanoylossus. Young buds of 

 Cephalodiscus show a distinct division of the body into three consecutive 

 regions, comparable to the proboscis-, collar-, and trunk-regions of Balano- 

 glossus. These regions correspond to a similar number of sections in the coelom, 

 the proboscis -cavity being simple and unpaired, while the cavities of the 

 collar and the trunk are each divided by a mesentery into paired halves. In 

 the adult Cephalodiscus, the proboscis - cavity is restricted to the interior 

 of the large oral disc, which lies like an epistome above the mouth. This 

 cavity, according to HARMER, opens externally through two pores (proboscis- 

 pores) which perforate the anterior part of the nervous system (Fig. 42, ex). 

 These pores would correspond to the proboscis-pore or pair of pores (B. Kup- 

 fferi, BATESON) of Balanoglossus. According to EHLERS, on the contrary, 

 these pores represent the aperture of a special excretory organ, consisting of 

 a canal (Fig. 42) which passes over into a wide terminal section lined with 

 epithelium. The second section of the body- cavity also opens externally 

 through two pores (collar-pores) beneath the apron-like fold, known as the 

 operculum, which hangs down behind the mouth. To this region of the body 



* [In the foregoing account of the Bryozoa, considerable stress has been laid 

 on the supposed relationship of the Ectoprocta to Phoronis. Such a connection 

 is by no means recognised by all students of the Bryozoa. HARMER (Ectoprocta 

 Lit., No. III.) does not consider that the Ectoprocta have any connection with 

 Phoronis, and he would regard any structural resemblances as the result of 

 coincidence rather than of close relationship. Both HARMER and PROUHO 

 (Ectoprocta Lit., No. V.) regard the Ectoprocta and the Entoprocta as nearly 

 related. ED.] 



f Cf. SPENGEL'S monograph on the Enteropneusta, p. 753, etc. Prof. 

 SPENGEL was kind enough to show us a number of drawings made from his 

 preparation of Cephalodiscus. These confirmed in all essential points the 

 statements of HARMER. 



