284 



PHYLUM ANNELIDA 



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 ca?lomic 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 PriapulidEe 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. 



MOLLUSCOIDEA 



The three classes — Phoronoidea, Polyzoa or Bryozoa, and Brachio- 

 poda are sometimes grouped under the old, not very happy, term 



RIOLLUSCOIDEA. 



The Molluscoidea are characterised by the presence of a true 

 coelom, 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. 



The development is in most cases insufficiently known, and it is 

 probable that further knowledge of it will remove these sets of animals 

 from their apparently anomalous position. 



