INVERTEBRATES OF UNCERTAIN POSITION 177 



The Dicyemid^e (Fig. 114) and Heterocyemid^e are para- 

 sites in the kidneys of Cephalopoda (cuttlefishes and octopods). 

 The Orthonectid^e (Fig. 115) are parasites in Ttjrbellaria 

 (Chap. VII), Nemertinea, Annelida (Chap. XI), and brittle- 

 stars (Ophiuroidea, p. 199). 



2. Nemertinea 



The Nemertinea (Gr. nemertes, true) (Figs. 116, 117) have a 

 superficial resemblance to flatworms and are by some authorities 

 placed in the Phylum Platyhelminthes either as a distinct class 

 or as a supplementary group. Some of them are very long, 

 reaching a length of ninety feet. A few species live in moist earth 

 and fresh water, but most of them are marine. Cerebratulus 



Fig. 116. — Micrura verrilli, one of the Nemertinea found on the 

 Pacific coast. (From Weysse, after Coe.) 



(Fig. 117) and Micrura (Fig. 116) are marine; Geonemertes and 

 some species of Tetrastemma are terrestrial; and Malacobdella 

 is a parasite in certain mollusks. 



The most important anatomical features of the Nemertinea 

 are the presence of: (1) a long proboscis (Fig. 117,2, 10), which lies 

 in a proboscis sheath just above the digestive tract, and may 

 be everted and used as a tactile, protective, and defensive organ; 

 (2) a blood vascular system consisting usually of a median dorsal 

 and two lateral trunks (Fig. 117, 9); and (3) an alimentary canal 

 with both mouth (Fig. 117, 7) and anal openings. The blood 

 vascular system is here encountered for the first time. Nemer- 

 tinea possess a mesoderm and nervous and excretory systems which 

 do not differ markedly from those of the flatworms. The pro- 

 boscis sheath may represent the ccelom, but this is not certain. 



N 



