vi NEMATODA CLASSIFICATION I 37 



the Polymyarii, in which numerous muscle cells are seen in a 

 transverse section ; (ii.) the Meromyarii, in which only eight are 

 seen, two in each quadrant ; and (iii.) the Holomyarii, in which 

 the muscles are either not divided, or only divided by longitudinal 

 lines. This grouping has, however, to some extent broken down, 

 since Butschli 1 and others have shown that the third subdivision 

 is founded on insufficient observation, whilst the first two include, 

 in different subdivisions, Nematodes which are closely allied in 

 all respects except as regards their muscle cells. 



The details of the life -history have been used by other 

 writers as a basis of classification. Linstow 2 enumerates fourteen 

 distinct modifications of the post-embryonic development (vide p. 

 159), and Orley 3 has grouped these under three headings. The 

 animals which fall under each group to some extent resemble one 

 another in structure. Orley's groups are : 



(i.) Nematozoa. Thread-worms with free larval life, the 

 mature forms being parasitic in animals. Enormous numbers of 

 eggs are produced, and the development is indirect. The genital 

 organs are complicated by many convolutions. 



(ii.) Rhabditiformae. Small, as a rule microscopic, thread- 

 worms, usually living free, but rarely parasitic. They become 

 sexually mature only in decomposing organic substances, or in 

 earth saturated with such substances. They live gregariously and 

 do not produce immense numbers of ova. The metamorphosis is 

 slight, or is complicated by sexual metamorphosis. The oesophagus 

 has two dilatations. The genital tubes are simple and not 

 coiled. 



(iii.) Anguillulidae. Small microscopic thread-worms, with a 

 free existence in mould or water, throughout all stages. They 

 produce large eggs. They are provided with a caudal sucker 

 and bristles, sometimes with eyes and other structures charac- 

 teristic of a free life. Genital tube simple and not coiled. 



The disadvantage of such a system is, that to accurately place a 

 specimen in its proper class we must be acquainted with its life- 

 history, and this is known in but few cases. 



The determination of the species to which a Nematode belongs 

 is a matter of considerable difficulty. Amongst the more important 

 features for purposes of classification are the arrangement of the 



1 Zeitschr. wiss. Zool. vol. xxiii. 1873, p. 402. Ibid. vol. xlii. 1885, p. 708. 



3 Ann. Nat. Hist. 5 th ser. vol. ix. 1882, p. 301. 



