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 differences 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 JM/alacobdella 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 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 Trematoda. 
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 Zrzstoma (R. An. pl. 36), 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 Planaris (R. An. pl. 37, jig. 1°) 
the gastro-vascular ramification forms a complete net-work, which has its 
counterpart in Rhizostoma (R. An. pl. 50). 
We lay no stress upon the absence of the articulate structure in most 
Helminthes, because it is wanting in some of the Epizoa, which, although 
sometimes arranged with the. Radiata, belong to the Crustacea. The 
articulate appearance of Tenia 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 1848, 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 On ea 
Sus-otass J. Hetmimnrnopryra. 
Animals simple or compound; form ribbon-shaped, a double alimentary 
canal, no vent. Including the two orders Cystica (from Kystis, a bladder) 
and Cestoidea (from estos, a band). 
a. Cystica, with one Sindy 
1. Hydatide. 
b. Cestoidea, with three families: 
1. Cysticercidee. 
2. Ligulide. 
3. Teeniide. 
Sus-ciass II. Parencuymara. 
Form flattened, rarely cylindrical, nervous system bi-radiate ; a bifurcated 
or branched alimentary sac. It includes the two orders and families: 
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