TARDIGRADA. 541 



latter, and after further changes of cuticle becomes a fully-developed sexual 

 Pentastomum ta-nioides, which does not differ to any very marked extent 

 from P. denticulntum. 



In their general characters the larval migrations of Pentastomum are 

 similar to those of the Cestodcs. 



The internal anatomy of the adult Pentastomum, as well as the 

 characters of the larva with two pairs of clawed appendages, are perhaps 

 sufficient to warrant us in placing it with the Arthropoda, though it would 

 be difficult to shew that it ought not to be placed with such a form as 

 Myzostomum (rids p. 369). There do not appear to be any sufficient 

 grounds to justify its being placed with the Mites amongst the Arachnida. 

 if indeed the rings of the body of the Pentastomida are to be taken as 

 implying a true segmentation, it is clear that the Pentastomida cannot be 

 associated with the Mites. 



BIBLIOGRAPHY. 



(539) P. J. van Beneden. " Rechcrches s. 1'organisation et le developpement d. 

 I.inguatulcs." Ann. d. Sden. Not., 3 Ser., Vol. XI. 



(640) R. Leuckart. " Bau u. Entwicklungsgeschichte d. Pentastomen." Leipzig 

 and Heidelberg. 1860. 



TARDIGRADA. 



Very little is known with reference to the development of the Tardigrada. 

 A complete and regular segmentation (von Siebold, Kaufmann, No. 541) is 

 followed by the appearance of a groove on the ventral side indicating a 

 ventral flexure. At about the time of the appearance of the groove the cells 

 become divided into an epiblastic investing layer and a central hypoblastic 

 mass. 



The armature of the pharynx is formed very early at the anterior 

 extremity, and the limbs arise in succession from before backwards. 



The above imperfect details throw no light on the systematic position of 

 this group. 



Tardigrada. 



(641) J. Kaufmann. " Ueber die Entwicklung u. systcmatische Stellung d. 

 Tardigraden." Ztit.f. wiss. Zool., Bd. ill. 1851. 



Summary of Arthropodan Development. 



The numerous characters common to the whole of the 

 Arthropoda led naturalists to unite them in a common phylum, 

 but the later researches on the genealogy of the Tracheata and 

 Crustacea tend to throw doubts on this conclusion, while there 

 is not as yet sufficient evidence to assign with certainty a 

 definite position in cither of these classes to the smaller groups 

 described in the present chapter. There seems to be but little 



