1 1 2 Descriptions of Preparations. 



embodied in this Table can be illusti'ated by dissection of the readily 

 procui'able animals, Gammarus Pulex, Oniscus Murarius, and Peri- 

 2)laneta Orientalis. 



Much has been written as to whether twenty or twenty-one is the 

 ty|}ical number of segments in the Arthropoda. The great number of 

 homonomous segments which in Myriapoda are developed posteriorly 

 to their thoracic region, enables us to eliminate them from consideration, 

 except so far as the thoracic and cephalic segments are concerned ; but 

 in all other Arthropoda, with the exception of the Trilobites and Phyl- 

 lopoda amongst Crustacea, the number of actually, if not of homologically 

 distinct segments, appears to be very definitely limited. The typical 

 number of segments has been considered here as being twenty, in ac- 

 cordance with the views of Professor Huxley, and in opposition to those 

 of Professors Milne-Edwards and Van Beneden ; inasmuch as the ' telson' 

 or terminal so-called segment of the Crustacea does not appear to possess 

 the characteristics of a true segment. In the Sessile-eyed Crustacea, the 

 telson is, according to Mr. Spence Bate, 1. c. p. xxi., who however apjjears 

 to reckon it as making a twenty -first segment, " generally an abortive, 

 and frequently a rudimentary part ;" and in the Isopoda, with the excep- 

 tion of two genera, it is always fused with the preceding segment. With 

 one, or perhaps two exceptions, the telson never carries appendages, 

 " whereas it is a law common to all Crustacea, that every segment has 

 its appendage ;" and Rathke, from whom however Van Beneden differs, 

 describes it as being developed after the other segments, and from the 

 dorsal aspect of the body. 



Even if it should be proposed to regard it as representing in a rudi- 

 mentary form the very gi-eat or all but indefinite number of homo- 

 nomous segments which we meet with in Apus amongst Crustacea, and 

 in the Myriapoda, we should still be justified in eliminating it from the 

 number of the typical segments of Arthropoda ; that is to say, from the 

 number of segments to which some of the best marked representatives of 

 three out of the four great classes into which the sub-kingdom is divided, 

 can all alike be shown to conform. The history of the development, 

 and to a considerable though lesser extent, that of the comjiarative 



ganglion corresponding in position and connections to the azygos ' ganglion frontal e' 

 of Insecta and Myriapoda has been discovered by Leydig in Oniscus. In other 

 Crustacea, the nervus recurrens has no ganglion frontale developed, but appears to 

 take origin from the supra-oesophageal ganglia. Brandt has figured a lateral paired 

 system of sympathetic ganglia in this Crustacean, see Med. Zool., Bd. ii. Taf. xv. 

 fig. 27 ; but Leydig declares the structures thus described to be merely glands in 

 connection with the stomach. 



