LECTURES ON EMBRYOLOGY. 



75 



[PLATE XXXI INTESTINAL WORMS.] 



PLATE XXXLl INTESTINAL WORMS J 



young are InfUiOrum^. in others, the develop- 

 ment resembles more that of Crustaceans and In 

 sects, there being an animal layer formed upon the 

 lower side of the yolk sphere, which surrounds 

 gradually the yelk and encloses it, so that the 

 narle is dorsal. Such a growth has been observed 

 in a worm of the Leech family, which occurs in 

 Fresh Pond, (Plate XXXIII) as well as in a marine 

 V\'orm of the bay of Boston, belonging to the ge- 

 nus Pasithae. 



I wish only to make some remarks upon the va- 

 rious metamorphoses which the Worms undergo. 

 Among the Intestinal Worms we have forms which 

 are cylindrical, and which present no extreme di- 

 visions in the body (Plate XXXII, fig. C). 



We have others which are also cylindrical. (Pen- 

 tastoma, Plate XXXII, figs. A, B) but in which we 

 have transverse ridges. There are very numerous 

 forms of the kind, which are flattened as the 

 Tapeworm. We have others in which the differ- 

 ent parts of the body (Plate XXXI, fig. C,) differ 

 widely the Cysticercus. Tiiere are others in 

 which the articulations are srill more distinct, and 

 there are again others (Plate XXVI, fig. E) in which 

 the articulations are scarcely distinct at all, but 

 which constitute really compound animals, as 

 there are always two united together Diplozoon. 

 There are again others, which are flat, (Distorna, 

 Plate XXVI, figs. A, B, C, D) and entirely unartic- 

 ulated, unless we should consider as articulations 

 those folds on the margin, which can scarcely be 

 considered so; but owing to the arrangement of 

 their parts, particularly that of their nervous sys- 

 tem, we find that they must be referred to the clnss 

 of Worms. Indeed although these animals have 

 been placed in a special class, owing to the fact 

 that they are Parasites, they cannot be grouped to- 

 gether with all other Intestinal Worms, nor fnrm a 

 class by themselves. They have little in common 

 with other Parasites, but this mode of existence. 



la fact, Intestinal Worms constitute various types, 

 of which the main common trait of character is to 

 live upon other animals, rather than to resemble 

 each other in their structure. But between Planaria 

 (Plate XXX, fig. B) there is the most remarkable 

 affinity. This is a Distoma. (Plate XXVI, figs. C 

 D) an internal Parasite, and we find that every 

 thing agrees in the structure with Planaria (Plate 

 XXX, fig. B). There is an alimentary canal, first 

 a simple tube, which divides afterwards into two, 

 and from which arise innumerable branches rami- 

 fying in the substance of the animal. 



The same structure exists in Planaria, an animal 

 which has been referred to another class, but the 

 resemblance is so great that it is now no longer 

 possible to separate them ; and very recently, Mr. 

 Blanchard has proposed to combine them, under 

 the name of Aneurosi ; and previously Professor 

 Owen had intimated the propriety of uniting them 

 with those broad Intestinal Worms. Their ner- 

 vous system agrees m6st remarkably, and agrees 

 not only with that of other Intestinal Worms, but 

 when properly understood, shows that the nervous 

 system of the Intestinal Worms, though seemingly 

 so peculiar, is reallv constructed upon the same 

 plan as that of other Articulata in general. In Ar- 

 ticulata in general, the nervous system consists of 

 a series of swellings, as I have shown before (Plate 



[PLATE XXXni NERVOUS SYSTEM OF WORMS.] 



XXXIII, fig. A). In Malacobdella (Plate XXXIII 

 fig. B), and in all intestinal worms, the nervous 

 system consists of a main mass about the alimen- 

 tary canal, and two longitudinal threads extending 

 along the two sides of the body, from which arise 

 other threads. We have now only to conceive that 

 the two parallel threads are brought nearer to- 

 gether, and combined in one continuous thread by 

 transverse commissures, to have the same uniform 

 system, which characterizes the higher Articulata in 

 which those swellings are combined. We have 

 again in Planaria the nearest possible approach to 

 the nervous system of the Intestinal Worms, which 

 really brings them much closer than they could be 

 brought before, and combines them all into one 

 class. 



The manner in which these animals are found is 

 very remarkable. The Distoma, as we have it 

 here, (Plate XXXIV, figs. 2.3.4) lives as a parasite 

 in the cavity of other animals upon their liver is 

 very frequently met with in the cavities of higher 



