HELMINTHES. 43 



inter-clnss with characters common to both. Milne Edwards, Gervais, and 

 Agassiz, think thej form a common type with the Articuhita, thns uniting 

 all the worms in one division. The latter disposes of the ditierences in the 

 nervous system, by considering them essentially the same, the two distinct 

 or bilateral threads (and sets of ganglia, where they exist) being approximated 

 to form a single series. The genus Malacohdella has certain intermediate 

 characters, which render it difficult to place, the nerves being separated, the 

 intestine simple, and the sexes separate. It is about an inch long, white, 

 translucent like Planaria^ and has a posterior sucker. It is marine, and 

 lives as a parasite in the mantle of the molluscous genera Mija and Yenus. 

 Blanchard thinks it forms a distinct type of worms, as the generative organs 

 alone have an affinity with those of the Annelida ; and Duvernoy places it 

 among the Tiematoda. 



On the other hand, the Ilelminthes may be regarded as a two-rayed animal 

 (a view taken in part by Duvernoy), allied to the Radiata by this very 

 nervous system, which, in Tristoma (H. An. jpl. 3G), forms a complete 

 circle, which may be compared with that of the Radiata. The digestive , 

 system of this animal and of Planaria, bears an analogy to that oi the 

 Radiata, even the more typical forms, for in the Echinidea it has already 

 departed from the radiated type. In some Planarife (R. An.^;Z. Zl^fig. V) 

 the gastro-vascular ramification forms a complete net-work, which has its 

 counterpart in Rhizostoma (R. An. pi. 50). 



We lay no stress upon the absence of the articulate structure in most 

 Ilelminthes, because it is wanting in some of the Epizoa, which, although 

 sometimes arranged with the Radiata, belong to the Crustacea. The 

 articulate appearance of Taenia arises from the fact, that each joint is to a 

 certain extent a single individual, affording an analogy with the Radiata, 

 which is strengthened by the transverse nutrient tubes. 



Cuvier, Duvernoy, and others, place the Ilelminthes among the Radiata. 

 The classification of the latter, in 18-I8, is as follows, the vernacular 

 names (which have no authority in science) being rej^laced by systematic 

 ones. 



Class Helminthes, comprising the three sub-classes, Ilelminthophyta, 

 Parenchj-mata, and Cavitaria. 



Sub-class I. IlELMiNTnoPHYTA. 



Animals simple or compound ; form ribbon-shaped, a double alimentary 

 canal, no vent. Including the two orders Cystica (from lu/stis, a bladder) 

 and Cestoidea (from ITestos, a band). 



a. Cystica^ with one family : 



1. Hydatidse. 



h. Cestoidea.^ with three families : 



1. Cysticercidffi. 



2. Ligulidse. 



3. Tasniidse, 



Sub-class II. Pakenchymata. 

 Form flattened, rarely cylindrical, nervous system bi-radiate ; a bifurcated 

 or branched alimentary sac. It includes the two orders and families : 



247 



