794 CLASS V. ARACHNID A. 



tubes lying in the cephalo-thorax and the abdomen. In each 

 there is a median abdominal portion lying dorsal to the intestine 

 and an anterior portion forming a widely distributed network 

 of tubes packed among the acini of the liver. From these the 

 paired efferent ducts pass downwards to open by slightly pro- 

 jecting orifices on the posterior aspect of the operculum. The 

 relation of the ducts with the coelomic spaces of the embryo has 

 not been followed. 



The males are smaller than the females and are further dis- 

 tinguished by the hooked, not chelate, termination of the second, 

 or second and third appendages, a character which they acquire 

 on arriving at maturity. The spermatozoa have an oval head 

 and a flagellum. 



At the breeding season the animals (L. polyphemus) come 

 ashore in pairs, at high spring tides, the male clinging to the 

 carapace of the female. The eggs are fertilized after they have 

 been deposited in the series of hole which the female digs for 

 their reception. The point selected is near the upper limit of 

 high tide, and the mass of eggs produced is said to be about half 

 a pint in volume. L. longispinus of Japan appears to have 

 similar habits. 



In development a blastodermic area is formed on the ventral 

 surface of the yolk. A depression representing the blastopore 

 forms and lengthens into a " primitive groove," at the anterior 

 and posterior ends of which the mouth and anus are formed. 

 The mesoderm is formed by proliferation of the cells along the 

 primitive groove. Later it becomes segmented and coelomic 

 cavities appear by splitting of the mesoblast, in some of the 

 segments. The relation between the fifth pair and the excretory 

 organs is noticed above, and also the existence of a pair of meso- 

 blastic somites and of a pair of ganglia corresponding with the 

 chilaria, thus establishing their character as appendages. 



On escaping from the embryonic cuticle the young Limulus is 

 without an elongated caudal spine and swims freely by means of 

 its abdominal appendages, of which only three pairs have as yet 

 been formed (Fig. 516). With its marked lateral eyes, segmented 

 -abdomen and body divided into median and lateral regions by 

 longitudinal grooves, it presents considerable resemblance to a 

 Trilobite, and the stage has in fact been called the Trilobite stage. 

 The number of segments composing the anterior division of the 



