242 INVEKTEBKATA CHAP. 



PANTOPODA 



The Pantopoda, or sea-spiders, are treated by most zoologists as 

 a quite independent group of Arthropoda. Huxley, however, regarded 

 them as aberrant Arachnida. They agree with Arachnida in possessing 

 no proper jaws and in having four pairs of walking legs, but differ 

 from all Arachnida in the absence of the division of the body into 

 a prosoma with leg-like appendages in front, and an abdomen with 

 plate -like appendages behind, behind one of which the genital 

 ducts open. 



In the Pantopoda the so-called abdomen is an unsegmented stump 

 devoid of appendages, and the genital ducts, of which there may be 

 several pairs, open at the bases of the long legs. Further, the three 

 front segments bear pairs of legs reduced in size and not used for 



FIG. 191. Larva of Ascwhynchus minutus. (After Hoek.) 

 chel, chelophore ; ov, ovigerous leg ; p, palpus ; stom, stomodaeal proboscis. 



walking ; behind these come four pairs of walking legs, but since the 

 fourth of these is the seventh appendage it cannot correspond to the 

 last walking leg of Arachnida, though it may correspond to the 

 suppressed segment of Limulus and the scorpion. 



The most plausible suggestion as to the origin of the Pantopoda 

 is that they represent a divergent branch of primitive Arachnida at 

 the time that these were separating from the common stock of all 

 Arthropoda. 



The development of these interesting forms is very imperfectly 

 known, but what little is known only whets the desire to know 

 more. Thus in Pallene, according to Morgan (1891), the egg exhibits 

 a complete segmentation, and a blastula results, surrounded by a few 

 large columnar blastomeres and provided with a small blastocoele. 

 Later, however, just as in BrancJiipus and Astacus, the inner ends of 

 these blastomeres coalesce to form an unsegmented mass of yolk. It 

 appears that the endoderm is formed by budding off cells into the 

 yolk, and it seems likely (though Morgan denies it) that this budding 



