AFFINITIES. NOMENCLATURE. 67 



system of tubes which are identified as vascular, and in all thero 

 is a curious organ in the base of the proboscis which consists 

 partly of the before -mentioned gut diverticulum and partly of 

 vascular and glandular tissue. This organ is called the central- 

 or proboscis-complex. A remarkable feature of the phylum 

 and one which it has in common with the Cephalochorda is the 

 absence or small amount of stellate or other connective tissue 

 cells between the coelomic epithelium and the epithelium of 

 the ectoderm and endoderm (p. 91). 



The group Enteropneusta was established by Gegenbaur in 

 1870 and was placed among the Vermes. They have been re- 

 garded by some authors as especially related to the Nemertines 

 and by others as possessing leanings towards the Annelids. 

 Metschnikoff * in 1869 pointed out their resemblances to 

 Echinoderms, while Gegenbaur (1874) and Huxley f (1877) 

 were the first to call attention to the Tunicate affinities, Huxley 

 going so far as to include the two in a new group, the Pharyngo- 

 pneusta. The connexion with the Vertebrata was suggested 

 by Sedgwick J in 1884, and their inclusion in the group Chordata 

 was accomplished by Bateson in 1885. 



The Echinoderm affinities first pointed out by Metschnikoff undoubtedly 

 exist and are dealt with below (p. 99) and in the section dealing with 

 affinities in the general account of Echinoderms. The suggested affinities 

 with Phoronis are discussed at the end of this chapter. 



The phylum is divided into two orders, the Balanoglossida 

 and the Cephalodiscida. The establishment of these orders has 

 been necessitated by the discovery of the genera Rhdbdopleura 

 and Cephalodiscus, which present all the important Enteropneust 

 features (except the gill apertures in Rhabdopleura] . 



A great variety of names has been proposed for the phylum here called 

 Enteropneusta. We have selected Gegenbaur's term on the grounds of 

 priority and usage. The discovery of Cephalodiscus and Rhdbdopleura, 

 and consequent necessity for the establishment of two orders within the 

 phylum, have induced some authors to restrict the term Enteropneusta 

 to the forms which we have here included under Balanoglossida, and to 

 employ another name, Hemichordata, for the phylum. We object to this 

 terminology for two reasons : in the first place Cephalodiscus is as much 



* Ueber Tornaria, Nachr. K. Oes. Wiss. u. Univ. Gottingen, 1869, p. 287- 

 292, and Z. /. w. Z., 20, 1870, pp. 131-144. 



t Journal Linn. Soc., Zool., 12, 1877, pp. 199-226. 



J Q.J.M.S., 24, p. 70. 



Q.J.M.S., 25 sup., p. 111. 



