394 ANNELIDA. 



splanchnic layers, and give rise to the rudiments of the two seg- 

 mental organs ; while the oesophagus arises as an invagination of 

 the ectoderm, and a postoral circle of cilia is formed around its 

 opening (fig. 318). The serous membrane is cast off with the egg 

 membrane, and the larva then contains all the essential organs of 

 the adult Sipunculus except the ventral cord and the blood-vessels. 

 At a later stage, during the growth of the larva, the ventral cord 

 is developed from the ectoderm, the circle of cilia disappears, the first 

 tentacles sprout out at the edge of the mouth, and the metamor- 

 phosis of the free-swimming larva into the creeping young Sipun- 

 culus is completed. 



Fam. Sipunculidae. Body elongated and cylindrical, the anterior part re- 

 tractile. The mouth is surrounded with tentacles, and the anus is dorsal. The 

 intestine is coiled spirally. Sqjunonlus nudus'L., Mediterranean. Phase olosonia 

 lave Kef., Mediterranean. Ph. elongatum Kef. St. Vaast. 



Fam. Priapulidae. Anterior part of the body without circle of tentacles. 

 Pharynx armed with papillae and rows of teeth. Anus at the posterior end of 

 the body and slightly dorsal, above it there usually projects a caudal appen- 

 dage which bears papilla-like tubes (branchiae). The intestine is straight. 

 Priapulm caudatus 0. Fr. Miiller. Halicryptus spinulosus v. Sieb., Baltic, 

 Spitzbergen. 



Sub-class 3. HIRUDINEA*=DISCOPHORA, LEECHES. 



Body either with short rings or not ringed, without parapodia, icith 

 terminal ventral sucker, hermaphrodite. 



The body of the Hirudinea, so far as its external form is con- 

 cerned, recalls that of the Trematoda, with which group the Hirudinea 

 have often been incorrectly connected. 



Externally the body is marked by a number of transverse rings, 

 which are short and may be more or less indistinct or even entirely 

 absent. These rings correspond in no way with the internal segments, 

 which are separated by transverse partitions or dissepiments; but 

 they constitute much shorter portions of the body, four or five of them 

 corresponding to one internal segment. The large sucker at the 

 posterior end of the body serves as an organ of adhesion ; and there 

 may be in addition a second smaller sucker, either in front of or 



* Brandt and Ratzeburg, " Medicinische Zoologie," 1829. Moquin-Tandon, 

 " Monographic de la famille des Hirudinces," 2nd. edit., Paris, 1846. Fr. 

 Leydig, " Zur Anatomic von Piscicola geometrica," Zeitscltr.fiir tviss. Zool., Tom. 

 I., 1849. H. Rathke. " Beitrage zur Entwickelungsgeschichte des Hirudineen," 

 edited by R. Leuckart, Leipzig, J862. R. Leuckart, "Parasiten des Menschen," 

 Bd. I., Leipzig, 1863. Van Benedenet Hesse, " Recherches sur les Bdelloides ou 

 Hirudinces et les Trematodes marins," 1863. Robin, " Memoire sur le developpe- 

 inent embryogenique des Hirudinees," Paris, 1875. 



