TYPE PLATYHELMINTHES. 167 



3. Order Hoplonemertini. 



This order, wliicli includes the genera Tetrastemma and 

 Amphiporus mentioned above, has, like the preceding order, 

 ciliated funnels as lateral organs, and the nervous system lies 

 completely within the muscular layer of the body-wall and 

 the nerve-cords are united by transverse commissures, the 

 plexus being wanting. The blood vascular system is a closed 

 series of tubes, not communicating with lacunar spaces. The 

 most striking characteristic of the order is, however, the 

 structure of the proboscis, which is armed near its posterior 

 (that is, while invaginated) end by one or more dagger-like 

 spines or stylets. The most posterior portion is not capable 

 of being evaginated, and its walls are glandular, secreting a 

 poisonous fluid which is poured into the more anterior por- 

 tion of the tube, bathing the stylets and thus being carried 

 into the wound which may be made by the forcibly evaginated 

 proboscis with the stylets coming into contact with the body 

 of the prey or enemy. 



4. Order Malacobdellina. 



This order contains a single genus, Malacobdella, which is 

 found in the mantle-cavity of marine Lamellibranchs, such as 

 the common Mussel and Clam. It resembles the Hoplonemer- 

 tini in many particulars, but is destitute of lateral ciliated 

 organs, and its proboscis possesses no stylets. The intestine 

 is a convoluted tube without lateral diverticula, and the hind 

 end of the body is provided with a sucker. 



Development of the Nemertina. — In some Nemerteans, such as Tetra- 

 stemma and Malacobdella, the young worm leaves the egg in the form of a 

 cylindrical ciliated larva, usually provided at the extremities of the body 

 with bunches of longer cilia, which may possibly be sensory in function, 

 and gradually changes without any marked metamorphosis into the adult 

 form. The mouth opens upon the ventral surface of the body into a re- 

 tort-shaped digestive tract which in early stages possesses no anus — ^this 

 structure only appearing much later. In many forms, however, a peculiar 

 metamorphosis occurs during the transformation of the larva, known from 

 its first describer as Desor's larva, into the adult. On the ventral surface 

 of the body there appear four invaginations of the ectoderm, two situated in 



