LECTURES ON EMBRYOLOGY. 



69 



proportionably the greatest appendage, the longest 

 and most developed part of the body, in the earli- 

 est condition, being now reduced to the simplest 

 and lowest condition. 



Such a classification agrees with the classifica- 

 tion which has been introduced into our natural 

 histories, from tfye general impression received 

 from these animals. Guided to some extent by 

 anatomical details, and also in some points by em- 

 bryonic data, the arrangement proposed has been 

 the same to which, from embryonic evidence, we 

 would arrive. Only, there is an objection to be 

 made to the division of Crustacea into two groups; 

 Entomostraca, passing by transformation into Ma- 

 lacostraca, as can be directly ascertained in the 

 case of Cuma,the young Pctlaemon. Therefore, that 

 division cannot stand as a natural division. We 

 must have a series of groups following each other, 

 according to their embryonic gradation, but not 

 two types of Crustacea; as the diflerences upon 

 which this distinction rests present only degrees of 

 one and the same thing. 



But, there is another point in which the analogy 

 of gradation with embryonic growth is most re- 

 markably striking. It is the order of succession 

 of Crustacea in geological times. Crustacea 

 have existed from the earliest times. They are 

 found in the earliest formations, and found in all 

 subsequent beds. 



[PLATE XXTIt TRILOBTTE 1 



The forms assumed are different. The oldest are 

 the so-called Trilobites of several types (Plate V). 

 There is a remarkable analogy between the forms 

 of various Trilobites, and the outlines of the germ 

 of Crustacea, as figured Plate III, the earlier stages 

 reminding us of Agnostus, and the like, whilst the 

 later agree more with the higher Trilobites; but the 

 most striking resemblance is noticed on comparing 

 these types with the embryo of the Entomostraca, 

 as they are represented (Plate XX, fig. A) within 

 the egg, before they are hatched ; the divisions of 

 the middle part of the body into three lobes, the 



long, lateral appendages arising from the anterior 

 extremity. Every point of the structure agrees. 

 It is only, that in these ancient types there was a 

 permanent state of growth a condition under 

 which this animal lived for ages, and reproduced 

 its species; whereas, in our lowest Crustacea we 

 find even such an arrangement in the ea;lier form 

 only, as the beginning of a metamorphosis. 

 Next, we have in the geological series, Horse-Shoe 

 Crabs. During the coal period, there existed seve- 

 ral genera of Crabs allied to the Horse Shoe,having 

 the same general features. There are also species 

 found in the Oolitic beds. If we trace the grada- 

 tion of types, we find that these (Plate XX fig. A) 

 the Apus, in their perfect state, are next in order. 

 Those which undergo a retrograde metamorphosis 

 or which agree with the embryonic stage of Apus, 

 as Trilobites, being altogether the 'owest. And so 

 we have the Horseshoe Crab, which is the second 

 type in the order of geological ages, ranking high- 

 est among Entomostraca; taat is,above those which 

 resemble the Trilobites. 



During the deposition of the Oolitic and Creta- 

 ceous rocks, there existed a countless number 

 of Crustacea, but all of them were Lobster and 

 Shrimp-like animals. The earliest of all the Mala- 

 costraca is a long tailed animal, the Palinurus 

 Sueurii, resembling Lobsters and Shrimps. And 

 during all this time, we have only such animals 

 and not one Crab is formed until afterwards. But, 

 during the later part of the deposition of chalk,we 

 begin to find Crustacea with short tails, belonging 

 to the type of Crabs. So that, in the order of suc- 

 cession of the more recent types, we have the same 

 evidence that the arrangement which is proposed, 

 from embryonic data, is also the order of progress 

 which has been introduced into the character of 

 these animals at different successive periods. 



And I may add here, that the geographical dis- 

 tribution corresponds even to this gradation of 

 types, as far as it is understood. Crabs, for in- 

 stance, are not numerous on this shore. Few spe- 

 cies occur here. In the Middle States they are 

 more numerous. They occcur more frequently 

 and are very diversified in South Carolina ; and 

 still more numerous, in the tropics, where Crabs 

 prevail over Lobsters and Shrimps. And, though 

 these latter are extensively found in temperate 

 regions, it may be said, that the lower orders of 

 Crustacea (Plate XVIII, fig. A) are innumerable in 

 the northern regions, and much fewer in the trop- 

 ical regions. So that, in whatever point of view 

 we notice this subject, we see one plan, one com- 

 bination, one system, uniformly carried out. 



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