SIPUNCULIDS AND PRIAPULIDS. 235 



Echiuridas are apparently modified Chaetopods, while the position of 

 the Sipunculidaa and Priapulidoe is quite uncertain. 



Both include marine worms, living in the sand or mud upon which 

 they feed, having unsegmented bodies with a capacious body cavity, 

 and an anterior protrusible proboscis or introvert, which is moved by 

 special retractor muscles, and bears the mouth at its tip. In most other 

 respects the two classes differ markedly from one another. 



In the Sipunculids, the large introvert terminates in a hollow 

 tentacular fringe, within the cavity of which closed blood vessels run. 

 The gut is much coiled, and the anus is dorsal and anterior. A nervous 

 system with a distinct brain, a gullet-ring, and a ventral cord is present, 

 but the ventral cord is unsegmented. Large nephridia or brown tubes, 

 usually two in number, occur in the anterior region, and function also 

 as genital ducts. The sexes are separate, and the reproductive cells 

 develop on the lining of the body cavity. In the development, which 

 includes a metamorphosis, several peculiarities are observable, tending 

 to show that the animals are not primitive. The larva of Sipuncuhts 

 is sometimes compared to a trochosphere, but differs from a typical 

 trochosphere, notably in the total absence of segmentation, of "head 

 kidneys," of a pre-oral band of cilia, as well as in the position of mouth 

 and anus, and the slight development of the pie-oral lobe. 



The class includes eleven genera, which are widely distributed ; many 

 of the species are large and conspicuous. It should be noticed that 

 while Sipunculids are typically without trace of setae, some genera, 

 e.g. Phascolosoma, have distinct hooks on the introvert. 



The Priapulidse include two genera Priapitliis and Halicryptus, both 

 almost entirely confined to the northern hemisphere. They have no 

 tentacles, no vascular system, no brown tubes, and no brain. The gut 

 is straight, or has a single loop ; the anus is posterior. A gullet-ring and 

 ventral nerve-cord are present as in Sipnnciilus, but retain their 

 primitive connection with the epidermis. There are complex genital 

 ducts opening by a pore on each side of the anus, which in the young 

 are connected with an excretory system of the Platyhelminth type, 

 while in the adult they are overgrown and concealed by the repro- 

 ductive cells. The development is unknown. In Priapulits there is a 

 peculiar respiratory (?) appendage at the posterior end of the body. 



Appendix (4) to Annelid Series. 



Under the old term Molluscoidea are sometimes included the three 

 classes Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda. 



The Molluscoidea are characterised by the presence of a true 

 ccelom, formed in development by the folding off of pouches from 

 the archenteron, and by the shortening of the dorsal region of the 

 body, which results in the close approximation of mouth and anus. 

 The mouth is typically furnished with ciliated tentacles, and is often 

 overhung by an epistome ; both tentacles and epistome, when present, 

 contain spaces which are part of the body cavity. Except in the 

 Ectoprocta among Polyzoa, two or four nephridia are present, and 

 serve also as genital ducts. There is always a metamorphosis in 

 development, and the larva; are peculiar. 



