HELMINTHES. 43 



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

 Agassiz, think they form a common type with the Articulata, thus 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 Planarla^ and has a posterior sucker. It is marine, and 

 lives as a parasite in the mantle of the molluscous genera Mya and Venus. 

 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 Ti-ematoda. 



On the other hand, the Helminthes 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 Trlstonia (R. An, pi. 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 of the 

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

 departed from the radiated type. In some Planariie (R. An.^?Z. ^l.,Jig. V) 

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

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



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

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

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

 articulate appearance of Tiienia 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 Helminthes among the Radiata. 

 The classification of the latter, in 18i8, is as follows, the vernacular 

 names (which have no authority in science) being replaced by systematic 

 ones. 



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

 Parenchymata, and Cavitaria. 



Sub-class I. Helminthophyta. 



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

 canal, no vent. Including the two orders Cystica (from Kystls., a bladder) 

 and Cestoidea (from Kestos., a band). 



a. Cystica^ with one family : 



1. Hydatidae. 



h. Cestoidea, with three families : 



1. Cysticercidse. 



2. Ligulidas. 



3. Tteniidse. 



Sub-class II. PxIeenchymata. 

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

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



247 



