214 



ARACHNIDA. 



Fig. 110. 



d. 



very important pecu- 

 liarity, but which was 

 in some measure in- 

 dicated in the pre- 

 ceding period. Its 

 size is slightly di- 

 minished anteriorly, 

 and the vitellus con- 

 sequently is divided 

 e into two portions. 

 The smaller and an- 

 terior part (a) is rea- 

 dily distinguishable 

 from the dorsal part 

 of the foetus, and occupies the place which sub- 

 sequently becomes that of the corslet ; M. 

 Heroldt consequently terms it the thoracic re- 

 gion. The other part is the abdominal region, 

 which is very conspicuous, occupies more than 

 one-half the bulk of the ovum, and seems to 

 constitute the greatest portion of the abdomen. 

 If the inferior surface of the abdominal region be 

 examined, there will be seen, in addition to a 

 spot which ornaments that part, some addi- 

 tional oblique and curved folds, which indicate 

 the formation of the integuments ; another and 

 a more important change has now taken place 

 on the middle line of the superior surface; viz. 

 an obscure straight band (b) which commences 

 at the thoracic-abdominal constriction, and 

 reaches to the extremity of the ovum, becoming 

 gradually narrower in that direction. This 

 band, which does not give off lateral processes 

 in any part of its course, is to be considered as 

 the rudiment of the heart or dorsal vessel. The 

 fluid which it doubtless contains in its interior 

 is motionless. Heroldt thinks that the forma- 

 tion of the fluid is anterior to that of the 

 parietes in which it is enclosed : he also be- 

 lieves that it is the albumen which gives origin 

 to the circulatory apparatus, and further attri- 

 butes to it the origin of all the integuments. 

 These are, doubtless, important questions to 

 solve, but as they are the result of speculation 

 rather than direct observation, we have deemed 

 it proper to omit the theories by which they are 

 supported, and confine ourselves to a simple 

 enunciation of the facts The eyes (d) are 

 now distinguishable. 



Ninth period. The ovum presents a more 

 sensible diminution anteriorly, and is more dis- 

 tinctly divisible into two parts. The anterior 

 and narrow portion constitutes the smaller ex- 

 tremitv, and includes the head, the thorax, and 

 their appendages ; the other portion, which is 

 spherical and of much larger size, constitutes 

 the greater extremity and corresponds to the 

 abdomen. At the same time that these modi- 

 fications take place the ovum becomes slightly 

 elongated, and a'l the parts which can be dis- 

 tinguished therein have proceeded towards 

 their perfection. The legs now present slight 

 traces of a division into joints, and they have 

 increased so far in length that they cover almost 

 the whole of the lower surface of the thorax. 



Tenth period. The small extremity, which 

 is still more elongated, is now found to be dis- 

 tinguished from the large one by a true con- 

 striction, dividing the ovum into the parts de- 



nominated in the perfect spider ' thorax' and 

 ' abdomen.' The visible parts of the thorax 

 are the mandibles, the palpi, and the legs ; 

 these latter appendages are folded upon the 

 chest, and have grown so long as to cross the 

 middle line of the body; they are locked in the 

 interspaces of each other, like the fingers when 

 the hands are clasped together. The abdomen 

 presents nothing remarkable, except the elon- 

 gated opake streak which exists along the middle 

 of the inferior surface from the feet to the termi- 

 nation of the abdomen, and which was already 

 visible at the preceding periods. (Fig. 110, e.) 

 Heroldt imagines this streak to be an indication 

 of the development of the internal parts of the 

 abdpmen, viz. the intestinal canal, the secreting 

 vessels of the web, and the genital organs, &c. 

 In proportion as the fetus increases, the ex- 

 ternal membrane or covering; of the egg is ap- 

 plied more exactly to its body, and seems to 

 represent an exterior skin, of which the young 

 spider soon divesls itself, almost in the same 

 manner as the caterpillar sheds the skin in 

 which it is enveloped. 



Eleventh period. By the progressive in- 

 crease of the foetus the membrane of the egg 

 becomes so much stretched, and is applied so 

 exactly to the surface of the body of the animal 

 that the different parts can be distinctly seen 

 through it, like the nymph or chrysalis of certain 

 coleopterous insects. The essential parts of the 

 thorax are the head and the feet. The head is of 

 a white colour, and is surmounted by eight 

 brown streaks ; the legs, which are also white, 

 are closely applied to the chest, with their extre- 

 mities alternating with each other. One may dis- 

 tinguish in each a hip,a thigh, a leg, and a tarsus. 

 The articulations of the palps and mandibles 

 are also visible through the general envelope of 

 the egg. The inferior streak of the abdomen 

 is much more extended, and seems to be divi- 

 ded into two parts, one large and elliptical in 

 figure, the other small and rounded ; the latter 

 corresponds to the anal aperture; at this last 

 stage of the development, the fetus or the im- 

 prisoned young spider, as it may be called, 

 gives no sign of motion. 



Etc/union or hatching of the spider. At 

 length the spider bursts the egg by tearing 

 through the exterior membrane. De-Geer* has 

 described this phenomenon. The outer mem- 

 brane or pellicle of the ovum becomes fissured 

 along the corslet, and the spider protrudes by 

 this aperture, first the head, the mandible?, the 

 thorax, and abdomen, after which there remains 

 the more difficult operation of extracting the 

 legs and maxillary palps from that part of the 

 outer membrane with which these parts are, as it 

 were, enveloped. This is at length effected, 

 though slowly, by alternately dilating and con- 

 tracting the body and legs, upon which the 

 animal is liberated, and capable of progression. 

 In proportion as the parts are disengaged from 

 the pellicle, it is pushed towards the extremity 

 of the legs, and is reduced to a little white bag 

 which is all that remains. Sometimes the pel- 

 licle is found still slightly adherent to the ab- 



* Mem. sur Its Inscctcs, t. vii. p 195. 



