216 THE ANATOMY OF INVERTEBRATED ANIMALS. 



layer of circularly disposed muscular fibres may be added. 

 The oral end of the body may have the form of a retractile 

 proboscis (Priapulus), or be provided with tentacular append- 

 ages. These may be arranged in a circle round the mouth, 

 and short {Sipunculus, Fig. 56, I., T), or long (Phoronis), or 

 there may be a single long, sometimes bifurcated and ciliated, 

 tentacular appendage (Bonellia). Filamentous appendages, 

 which are probably branchiae, are given off at the hinder end 

 of the body in Sternaspis and Priapulus. The endoderm is 

 usually ciliated throughout. The intestine is straight in most 

 genera, but is coiled and bent upon itself, so as to terminate 

 in the middle of the body, in Sipunculus (Fig. 56, I.). In 

 Phoronis the anus is close to the mouth. The anal aperture 

 is always situated upon the dorsal aspect of the body. There 

 is a spacious perivisceral cavity, undivided by mesenteries, 

 which in some cases (Priapulus, Sipunculus) opens externally 

 by a terminal pore. In Echiurus, Bonellia, Thalassema, a 

 pair of tubular, sometimes branched organs, which are ciliated 

 internally, and communicate by ciliated apertures with the 

 perivisceral cavity, open into the rectum. These appear to 

 represent the water-vessels of the Botifera and the respira- 

 torv tubes of the Holothuriw. 



"A pseud-haemal system exists in most (Sipunculus, Sternas- 

 pis, Bonellia, Echiurus, and Phoronis), and, when fully devel- 

 oped, consists of two longitudinal trunks — one dorsal, or su- 

 pra-intestinal, the other ventral, with their terminal and lateral 

 communications. The pseud-haemal fluid is colorless, or may 

 have a pale reddish tinge, in most, In Phoronis it_ is said to 

 contain red corpuscles. In Sipunculus, the cavities of the 

 tentacles communicate with a circular vessel provided with 

 caecal appendages ; and this circular vessel is said to open 

 into the pseud-haemal vessels. 



The nervous system presents a collar, which surrounds the 

 oesophagus, and from which a simple or ganglionated cord 

 proceeds backward in the ventral median line, giving off lat- 

 eral branches. The ventral cord contains a central canal, and 

 the collar usually presents a cerebral ganglionic enlargement. 

 Rudimentary eyes are sometimes connected with the cerebral 

 ganglion. 



The sexes are distinct, and the reproductive elements are 

 developed either from the parietes of the perivisceral cavity 

 or in simple caecal glands. In Sipunculus, the ova and sper- 

 matozoa float freely in the perivisceral cavity. 



The actively locomotive embryo of Sipunculus (Fig. 56, II.) 



