232 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



ages and the general uniformity in plan, so far as the anterior part of the body is con- 

 cerned, in all Arachnida. But it may be asked, what becomes of the hinder thoracic and 

 the abdominal somites in the Spiders and Mites ? "Without, at present, giving a positive 

 answer to this question, I am inclined to think that the Spiders stand to the Scorpion in 

 the relation of Lcemodipoda to Amphipoda, and that many of their posterior somites are 

 aborted. 



I do not doubt that many minor variations wiU be detected when the morphology 

 of the Articulata is carefuUy examined ; but I venture to think it a result of no small 

 moment, if it can be proved that a Lobster, a Cockroach, and a Scorpion are composed 

 of the same primitive number of somites; that the head in each consists of the same 

 nmnber of parts, and that the great differences are the consequence of the different modi- 

 fication of the thoracico-abdominal somites, aU fotirteen of which bear appendages in 

 the Lobster, while only three (or if we consider the genital apparatus in the light of 

 appendages, five) are so provided in the Insect, and only two (leaving out of consideration 

 the "pectines") in the Scorpion. 



8. I have elsewhere* explained at length my views with regard to the nature of the 

 carapace in the Crustacea, and I will only repeat here, that there seems to me to be no 

 constancy in its composition. The rudimentary carapace of Squilla is assiu'edly deve- 

 loped from not more than io\xx somites, the antennary, mandibular, and maxillary. In 

 Apus, I doubt whether more than the six cephalic somites enter into its composition. 

 In Cuma it is constituted by the cephalic and three anterior thoracic somites, in Ilysis 

 by the cephalic and six or seven anterior thoracic, and in ordinary Fodophthalmia by .all 

 the cephalic and thoracic somites. 



9. Lastly, there are certain parts developed singly in the median line in the Articulata. 

 Of this natui'e are the frontal spines of Crustacea, their telson, and the sting of the 

 Scorpion, whose mode of development appears to be precisely similar to that of a telson. 

 In the same category we must rank the labrum in front of the mouth, which in the 

 Crustacea (at least) appears to be developed from the sternum of the antennary or third 

 somite, the metastoma (or so-called labium or lingua) of Crustacea, and the lingua of 

 Insecta, behind the oral aperture. 



However much these appendages may occasionally simulate, or play the part of, 

 appendages, it is important to remember that, morphologically, they are of a very ditferent 

 nature, and that the confusing them with true appendages must tend completely to 

 obscure the beautiful relations which obtain among the ditferent classes of the Articulata. 



§ 5. The Embryogeny of the Articulata, MoUusca, and Vertebrata compared. 



I find it difficult to conclude this memoir without saying a few words on the resem- 

 blances and differences between the embryogenic changes of the Articulata and those of 

 the MoUusca and Vertebrata. Absolute and fundamental differences appear to me to 

 separate the members of these three classes almost from the fii'st appearance of the germ. 



As we have seen, it is the neural side of the Arthropod which is first developed, while, 

 so far as I am aware, it is the opposite or haemal side which is first formed in every 



* "Lectures," Med. Times and Gazette, 1857. 



