SIPUNCULIDS AND PRIAPULIDS. 247 



Appendix (3) to Annelid Series 



A. Class SIPUNCULIDS, e.g. Sipunculus, and 

 B. Class PRIAPULID^E, e.g. Priapulus 



These two classes were formerly united with the Echiuridse as 

 Gephyrea, but it is improbable that the three are nearly related. The 

 Echiuridae are apparently modified Chsetopods, while the position of 

 the Sipimculid?e and Priapulidse 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. Peculiar ciliated vesicles or 

 "urns" arise in some Sipunculids as buds from the blood vessels, 

 and many swim freely in the body cavity. By collecting and agglutinat- 

 ing particles they help to purify the coelomic fluid. Large nephridia or 

 brown tubes, usually two in number, occur in the anterior region, and 

 function also as genital ducts. The sexes are separate except in 

 Phascolosoma minutum, and the reproductive cells develop on the lining 

 of the body cavity. In the development, which includes a meta- 

 morphosis, several peculiarities are observable, tending to show that 

 the animals are not primitive. The larva of Sipunculus 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 pre-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 Priapulidae include two genera Priapulus 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 Sipunculus, 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 Priapulus there is a 

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



