On the Assignment of the Class Pogonophora to a Separate Phylum of Deuterostomia Brachiata A. Ivanov, Phyl. Nov.

Abstract

JOHANSSON (9), having recognized that Lamellisabella belongs to a special class Pogonophora, has placed it close to the Phoronidea. However, the trunk of the latter represents a greatly extended ventral outgrowth of the larva and does not correspond to the trunk of the Pogonophora. Besides this, Phoronidea possess only two pairs of coeloms, gonads not separated from the coelom, and a pair of lateral nerve trunks. Much nearer to the truth is the opinion of Dawydoff (4) and Beklemishev (1) who place the Pogonophora near the Hemichorda. Now, when the organization of the Pogonophora is clear in its main traits, one can point out a series of characters in common with the Hemichorda. Such are: the composition of the body of three segments, the single anterior coelom, the pericardium, the coelomoducts of the first segment, the situation of the gonads in the trunk segment and their separation from the coelom, the dorsal nerve trunk and the epithelial position of the nervous system. Besides this, a peculiarity in common with the Enteropneusta is the secondary metamerization of the elongated trunk section, although in Pogonophora secondary metamerization affects also other organs. Notwithstanding this it would be wrong to include Pogonophora as a subphylum in the Phylum Hemichorda, as is done by Beklemishev (1). This is contradicted by the belonging of the tentacles to the first and not to the second segment as in Pterobranchia, by the strong development of the anterior coelomoducts which have an excretory function, by the absence of these organs in the second segment and by their development in the trunk section. Among the distinctive characters of Pogonophora are: the development of the brain in the first and not in the second segment, the development of the heart from the ventral and not from the dorsal vessel, and finally the strong cuticularization of the integument. The absence of gill slits and of a notochord cannot serve as distinctive characters because of the secondary reduction of the digestive tract. A series of peculiarities of the organization of Pogonophora developed under the influence of sedentary life. To these belong: the ability to secrete a protective tube, the extraordinary length of the trunk and the substitution of the digestive tract by a peculiar tentacular apparatus which performs the functions of accumulating, digesting and resorbing the food (7). All this forces us to regard the Pogonophora as an independent phylum of Deuterostomia which I propose to call Brachiata. The Phylum Brachiata may be characterized as follows. The body of these lower Deuterostomia consists of three segments of which the two anterior ones form by fusion the anterior section of the body. The first segment contains a single coelom with a pair of excretory coelomoducts, and forms the dorsal cephalic lobe and the tentacular apparatus. The second segment is without coelomoducts. The trunk segment is greatly elongated and contains a single pair of gonads; its coelomoducts function as gonoducts. The integument is cuticularized; ciliated epithelium is retained only partially. The brain belongs to the anterior segment; from it the dorsal nerve trunk originates. The entire nervous system occupies an epithelial position. The circulatory system consists of a dorsal and a ventral vessel connected anteriorly by a system of vessels of the tentacular apparatus. There is a muscular ventral heart which some-