600 COMPARATIVE ANATOMY chap. 



C. doclecalophus, the only known representative of the genus, was 

 found in the Magellan Straits at a depth of 245 fathoms. 



Systematic position. — Cephalodiscus shows in the following 

 points a remarka]:)le agreement with the Enteropneusta. 



1. The body falls into three sections (distinct even in the young 

 bud), one preoral and two postoral. The preoral section, the so- 

 called buccal shield, corresponds with the proboscis, the middle section 

 with the collar, and the larger posterior section and the pedicle with 

 the trunk of the Enteropneusta. 



2. These sections correspond with special sections of the co^lom, 

 an unpaired coelom in the buccal shield, and two pairs of co?loms in 

 the body proper. We recognise here the unpaired proboscidal ccelom 

 and the paired collar and trunk coeloms of the Enteropneusta. 



3. The pores of the coelom of the buccal shield correspond with 

 the proboscis pores of the Enteropneusta proper, which are also often 

 two in number. 



4. The pores of the pair of cceloms in the anterior body correspond 

 with the collar-pores. 



5. Cephalodiscus and the Enteropneusta have gill-slits, the former 

 having one and the latter many pairs. 



6. The anterior diverticulum of the buccal cavity corresponds 

 with the proboscidal diverticulum of the Enteropneusta. 



7. The central nervous system corresponds with the collar cord 

 (which, however, in this case is not sunk below the skin) of the 

 Enteropneusta and with its immediate continuation on to the base of 

 the proboscis. 



The differences existing between Cephalodiscus and the Entero- 

 pneusta may well be attributed, at least in part, to the tubicolous, half- 

 sedentary manner of life of the former. These are: (1) the anterior 

 position of the anus, and the consequent looped course of the 

 alimentary canal ; (2) the general crowding together of the most 

 important external organs (apart from the pedicle or stalk) at the 

 most anterior part of the body ; (3) the presence of a tentacular 

 crown, consisting of twelve feathered tentacles ; (4) the presence of 

 the pedicle or stalk ; ^ (5) the occurrence of asexual reproduction by 

 means of gemmation ; (6) the small number of gill-slits and genital 

 organs ; (7) the form of the body in general and especially that of the 

 proboscis ; (8) the absence of a blood vascular system. 



II. Rhabdopleura. 



This form, which was formerly classed with the Bryozoa,- is no 

 doubt somewhat nearly related to Cephalodiscus, but is further 

 removed than the latter from the Enteropneusta. 



'■ An apparently lioiuologous structure is, however, figured l)y Bateson on a young 

 Balanoglossus Kcnvalevskii. 



'^ In the first volume of this book, indeed, Rhabdopleura appears among the Bryozoa. 



