482 GENERAL HISTORY OF THE INrUSOETA. 



Sect. V.— OF THE TARDIGRADA. ' 



Their Structure, Habitats, and Affinities. — ^The Tardigrada or Tardi- 

 grades (in German, Wassei^bdren, lit. ivatei'-hears) constitute a small group 

 of animals, first noticed by Eichhorn, and latterly more fully investigated by 

 Doyere, Dujardin, and Kaufmann. 



Their size is so considerable (from -g^th to ^^^-th of an inch in length) that 

 they are visible to the naked eye. They have oblong, symmetrical, non- 

 ciliated, and very contractile bodies, admitting of their rolUng themselves into 

 a ball, and of otherwise varying their figure. The head is somewhat pro- 

 duced, assuming a conical or pyramidal figure ; but they have no pseudopo- 

 dium or other posterior process. 



They are invested by a resistant, firm, and sometimes horny integument, 

 composed of two layers. The firmness is due to the chitinous composition of 

 the external lamina or cuticle, which is not afiected by caustic alkali. In 

 Emydium, M. Doyere describes the integument to consist of foui' horny 

 plates. During contraction, the integument is thrown into transverse folds, 

 and the anterior and posterior segments retracted. Its smface is generally 

 smooth ; but in Emydium there are a few pretty regularly disposed bristles 

 (setse) on the back and sides ; and ia the neighbourhood of the mouth there 

 are, as a rule, several soft flexible processes, palpi or antennae. Numerous 

 and definite muscles extend between the inner skin or epidermis and the 

 various organs and members. 



The under or abdominal surface is clearly distinguished from the dorsal by 

 the presence of four pairs of rudimentary feet without joints, each consisting 

 of a nipple-like (mammilliform) process supporting on its extremity from two 

 to four well-developed curved and acute uncini or hooks. These are the 

 locomotive members by which the animals crawl upon and adhere to solid 

 substances. 



The head is without a trochal disk or ciliary wreath, vibratile ciha being 

 entirely wanting. The mouth, opening at its extremity, in the median line, 

 is modified so as to form a sucking-tube ; it is narrow, and drawn out to a 

 more or less fine extremity ; it is bounded on each side by a lateral, rigid, 

 horny, narrow or linear process — the maxilla, which is moveable upon a 

 single or double central piece or fulcrum. The whole organ constitutes a 

 tube -like sucker, and is protrusile at wiU beyond the head, like the suctorial 

 mouths of Aca?'i and Insecta. On each side of the mouth are the smaU re- 

 tractile palpi already noticed. 



The mouth opens posteriorly in a pharyngeal muscular bulb, furnished 

 internally with a horny articulated dental apparatus, serving to crush food, but 

 less highly organized than in Rotifera. Under the polarizing microscope the 

 manducatory organs exhibit the same appearance as horn. Erom them the 

 food passes into an elongated tubular stomach or intestine, continued straight 

 through the body, and terminating in an anus at the posterior extremity. In 

 its course it presents numerous lateral oifshoots or diverticula. 



No form of respiratory or circulatory apparatus has been detected ; but a 

 multitude of granules and corpuscles are seen to float freely in the general 

 cavity between the integument and the alimentary canal, which Doyere sup- 

 posed to be concerned in the processes of nutrition, and to be analogous to 

 blood-corpuscles. M. Quatrefages states that the fluid within the body is in 

 perpetual irregiilar motion. 



