TARDIGRADA. 163 



«ven as rudiments.* For this reason alone, the association of the 

 Tardigrada with the Arachnida and especially with the Acarina, 



which has repeatedly been attempted, chiefly on account of the 



number of limbs, cannot be maintained. With regard to the number 



of their limbs, the Tardigrada cannot be compared -with any other 



division of the Arthropoda, and the form of their limbs is so simple 



as to distinguish them in this respect also from all other Arthropoda. 



The segmentation of the body in the Tardigrada is peculiar, inasmuch 



as the abdomen is wanting and the anus lies in front of the last pair 



of limbs. In other points also the organisation of the Tardigrada 



shows peculiarities which distinguish it from that of other Arthropods; 



we may mention, by way of example, the unstriped muscle-fibres and 



the absence of special respiratory organs, and of a vascular system. 



For these and other reasons (cf. Plate, Xo. 3) we are led to consider 



the Tardigrada as a lateral branch of the Arthropod stock which 



separated from it near its root, and developed in a special and unique 



direction. 



LITERATURE. 



1. Doyere, XI. Memoire sur les Tardigrades. Ann. Sci. Nat. 



(2). Tom. xiv. Zool. 1840. 



2. Kaufmann, J. Ueber die Entwicklung und systematische Stel- 



lung der Tardigraden. Zeitsclir. f. Wiss. Zool. Bd. iii. 1851. 



3. Plate, L. Beitrage zur Xaturgeschichte der Tardigraden. Zool. 



Jahrb. Abth. f. Anat. Bd. iii. 1888. 



4. Siebold, C. Th. vox. Lehrbuch der vergleichenden Anatomie 



der wirbellosen Thiere. Berlin, 184S. pp. 552 and 553. 



APPENDIX TO LITERATURE ON TARDIGRADA. 

 I. Erlaxger, R. von. Beitrage zur Morphologie der Tardigraden. 

 Zur Embryologie eines Tardigraden, Macrobiotus macronyx. 

 Morph. Jahrb. 1895. And Biol. Centralbl. Bd. xiv. and xv. 

 II. Kexxel, J. vox. Die Yerwandschaftsbeziehungen und die 

 Abstammung der Tardigraden. Sitzwngsb. Ges. Dorp. 1891. 



* [Erlaxger (App. to Lit. on Tardigrada, Xo. I.) has made an exhaustive 

 investigation into the development of Macrobiotus macronyx. He finds that 

 cleavage is total and equal and results in the formation of a long, oval blastula 

 with a cleavage-cavity ; an iuvagination-gastrula arises whose blastopore, which 

 occupies the position of the future anus, soon closes ; a very short proctodaeuni 

 is formed, and the archenteron divides into a pharynx, gullet, and stomach ; four 

 pairs of enterocoeles form, and the embryo becomes divided into a head and 

 four thoracic segments. The head-coelom enters into connection with the first 

 pair of appendages ; the coelomic pouches of the second and fourth thoracic 

 segments enter into relation with the remaining appendages, while that of the 

 third segment gives rise to the gonads and (?) to a pair of enteric glands. The 

 oral papillae, the salivary glands, the nerve-ganglia, and the eyes all arise from 

 the ectoderm. Erlaxger regards the head and first two thoracic segments as 

 the cephalo-thorax ; the third and fourth segments as the abdomen, behind 

 which is a transitory post-abdomen. — Ei>.] 



