INVERTEBRATES OF UNCERTAIN POSITION 177 



The DICYEMID^; (Fig. 114) and HETEROCYEMID.E are para- 

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

 The ORTHONECTID^: (Fig. 115) are parasites in TURBELLARIA 

 (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 

 Paci&c 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: (i) 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, Q)\ 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 



