NEMEETEA 205 



lie in close relation to the dorsal cerebral lobes occur in most nemer- 

 teans: these are represented by a pair of lateral sensory grooves in 

 many paleonemerteans. In the hoplonemerteans and some heteronemer- 

 teans also occur one to three supraoral or frontal organs, which are 

 sensitive protuberances capable of being retracted so as to form a pit. 

 Simple ocelli, each with lens and nerve, occur in most nemerteans. 

 The number of such ocelli, which in a few forms are scattered along the 

 sides of the body, may exceed 200. A few species have auditory sacs. 

 The muscular system is well developed, consisting of two or three layers 

 of circular and longitudinal muscles which bear an intimate relation to 

 the main nerves. 



Most nemerteans are unisexual, but a few are hermaphroditic. The 

 gonads are paired, spherical organs, which usually lie between the intes- 

 tinal diverticula (Fig. 333) and discharge their products directly through 

 the body wall to the outside, no permanent genital ducts being present. 

 A few are viviparous. Development is direct in many forms, while in 

 others the young animal leaves the egg as a free-swimming larva 

 (pilidium or Desor's larva) and passes through a complicated metamor- 

 phosis before acquiring the form of the parent. 



History. The Nemertea have only quite recently been given an inde- 

 pendent systematic position, having been formally grouped with the 

 Turbellaria. The name originated with Cuvier, who in 1815 gave the ge- 

 neric name N emeries to Linens longissimus. Johnston (1846) first employed 

 the name for the entire group. At the present time it is a matter of dis- 

 pute whether the Nemertea should be grouped with the PlatJielmintJies or 

 be given an independent position. The modern classification is due princi- 

 pally to Hubrecht and Burger. The class contains over 400 species 

 grouped in 4 orders, 87 species having been found on our Pacific and 62 

 species on our Atlantic coast. 



Key to the orders of Nemertea: 



Oi No sucking disc present ; intestine not convoluted. 



&! Proboscis without stylets ; mouth behind brain ; intestinal caecum absent. 

 c x Muscular walls of body usually in 2 layers ; eyes usually absent. 



1. PALEONEMEBTEA 

 c a Muscular walls of body in 3 main layers ; eyes usually present. 



2. HETEBONEMEBTEA 

 5 2 Stylets usually present ; mouth in front of brain ; intestinal caecum 



usually present 3. HOPLONEMEBTEA 



a, Sucking disc present ; intestine convoluted 4. BDELLONEMEBTEA 



ORDER 1. PALEONEMERTEA. (PROTONEHERTINI; MESONEMERTINI.) 



Body long and slender, often filiform ; mouth usually far back, being 

 always behind the brain; proboscis without stylets; cerebral organ and 

 eyes usually absent; body wall contains two muscle layers, an outer 



