232 



LOWER INVERTEBRATES. 



Fig. 229. — Tornaria. 



Fig. 230. — Biilanoijlossits 

 minutus. 



and was long considered to be the lai-va of a starfish, until MetschnikofE established 

 its real affinities by tracing out its metamorphosis into the adult. The shape of the 



transparent larva is well shown in the 

 magnified drawing; it is just large 

 enough to be recognized by the naked 

 eye by those familiar with it. It may 

 be caught in July and August by skim- 

 ming the ocean surface with a fine net. 

 Much as it differs from the adult worm, 

 it yet passes by a series of gradual 

 changes into the mature animal, which 

 inhabits muddy bottoms between the 

 tides. Jialanoglossus is, I am con- 

 vinced, a modified annelid, although 

 its precise relationships are obscure, 

 especially on account of the singular- 

 ities of the nervous system. It has a 

 long, tapering, fragile body, the an- 

 terior lialf somewhat flattened, the posterior rounded; at the 

 anterior extremity is a large, pedunculated, top-shaped proboscis, 

 followed by a thickened ring or collar; behind the collar follow 

 the gills, a complicated set of bi-anchial openings, recalling some- 

 what the branchiae of the tunicates. Xow gills mark an advance 

 in organization from the \ermian towards the vertebrate type, 

 and Balanoglossus interests us scientificall}' just for this reason, that it appears in 

 some respects a connecting link between widely separated divisions of the animal 

 kingdom. 



Sub-Class IV. — Discophoei. 



The sucker-bearing annelids, or Hirudinei, or leeches, are segmented worms adapted 

 to a parasitic or semi-parasitic existence. They are all blood-suckers, veritable vam- 

 pires, and to most persons the mere thought of their habit is revoltino-; but the anti- 

 pathy they excite is not an altogether well-founded emotion, for they have their role 

 to perform, and man has converted them into his servants, and 

 given them a medical office, the duties of which they discharge 

 with praiseworthy alacrity. The leeches have been the theme 

 of one of the most singular of zoological delusions, in that they 

 have been considered to be related to the Tremadota. I know 

 no reason whatsoever for this queer conjunction, which is worthy 

 of the time when a whale was called a fish. It is true that both 

 leeches and flukes have suckers, but there all anatomical resem- 

 blance ceases. The leeches are true segmented worms, but the 

 transverse lines visible on the external surface of the body do 

 not mark off tlie segments, but, on the contrary, hide the true 

 joints ; hence their internal anatomy must be studied before the 

 true plan of their organization can be recognized. In general 

 shape we find that they are somewhat flattened ; that they are usually broadest poste- 

 riorly, tapering off rapidly towards the tail, slowly towards the head ; there is a sucker 



Fig. 231. — Saokei- and jaws 

 of leech. 



