188 



LECTURE XV. 



its station between the posterior : the eyes (a) now first make their 

 appearance. A cavity is formed behind the labrum, which communi- 

 cates with the commencement of the oesophagus ; the tail or abdo- 

 men (/) elongates^ and the depression in its surface is converted into 

 the anus ; the rest of the alimentary canal is a simple wide sac, which, 

 by the extension of the mucous layer of the germinal membrane, now 

 includes the vitelline mass. 



The three anterior pairs of maxillag begin to show themselves 

 at a little distance behind the mandibles, and afterwards the fourth 

 and fifth pairs ; the last {Jiff- 93. ff 5.) increasing in size more rapidly 

 than the rest. Thus, including the eyes (a) and the two antennae (b, c), 

 nine pairs of appendages may now be recognised, of which the two last 

 belong to the thorax : the five posterior pairs of thoracic members, 

 which are not, like the first two, developed into jaws, are produced in 

 regular succession from before backwards from that portion of 

 the body which is turned upwards, or the epimeral elements of the 

 rudimental segments. Each of the legs at its first appearance is 

 exactly similar to the hindermost maxillae ; these, therefore, are 

 retained in the service of manducation by an arrest of develop- 

 ment. The ambulatory legs increase inversely with respect to the 

 maxillae, the anterior {Jig. 93, h 1.) soon acquiring four times the 



length of the posterior (^ 5). The ru- 

 diments of the future branchiae next 

 appear as small processes from the base 

 of each leg. The seven segments of 

 the third division of the body (z) may 

 now be distinguished by six transverse 

 furrowSj and by the rudiments of foli- 

 aceous appendages. 



The heart (5) appears at first in the 

 shape of a small compressed vesicle, 

 situated near the junction of the an- 

 terior and posterior segments of the 

 body': blood-vessels seem to be prolonged from it, and its pulsation 

 speedily becomes distinguishable. 



The nervous system consists at first of eleven pairs of minute 

 white spots, from the anterior of which a short and broad process 

 passes forwards on either side of the oesophagus. The above described 

 stages of development are in progress from the beginning of April to 

 the middle of May. 



The whole of the organs continue to approach more nearly to 

 their mature form. The brain, liver {ii), and salivary glands (^v), next 

 make their appearance. The outer integument of the body is de- 

 veloped from the ventral to the dorsal aspect, and the yolk-laden 



Astacus fluviatilis. 



