398 



On Stigmata in the King Crab. 



[June 16, 



as it were, the gill-books ? " " Is there," we are led on further to 

 ask, " any known instance in Arachnida of the formation of cupped 

 area? on the chitinous surface of the body ? If so, can we show in 

 what mechanical relation they are formed ? And lastly, can it be de- 

 monstrated that such mechanical relation probably existed in con- 

 nexion with the gill-books of the assumed common ancestor of 

 Limulus and Scorpio ? " If all these questions can be affirmatively 

 answered, then our hypothesis as to the transition of the aquatic 

 Arachnid to the pulmonate condition acquires great plausibility. 



The answer to these questions, which I am able to give as the result 

 of some careful examinations of specimens of Limulus and Scorpio, 

 Thelyphonus, and other Arachnida, appears to me to have more than 

 ordinary interest, since the formation of cupped area? on the chitinous 

 surface of the body and the mechanical relations connected with their 

 formation have come to light as demanded by the hypothesis. They 

 exist in Limulus itself and in Thelyphonus. In Limulus, there are 

 two great muscles, a right and a left, inserted into the soft ventral 

 integument near the base of each double gill-plate. These muscles 

 known as the thoraco-abdominal muscles, serve (together with others 

 which enter the appendage itself) by their contractions to move the 

 gill-plates in the water and so aid in aquatic respiration. The position 

 of the insertion of each muscular mass is marked by a deep funnel- 

 like depression of the integument. From the external surface this 

 depression appears as a " stigma," and a rod can be introduced into 

 it to the depth of an inch. The funnel- like depression has a narrow 

 mouth which is often as much as half an inch in length. Internally 

 the invaginated cuticle stands up as a flexible chitinous tendon, giving 

 attachment to the muscle already mentioned. These structures 

 appear to have remained hitherto undescribed. 



In Limulus, I find a pair of these " muscle-stigmata," right and 

 left behind the genital operculum, and a pair (right and left) behind 

 each of the iamelliform fused appendages which carry the gill-books. 

 The mouth of each stigma is strengthened by a small " epistigmatic 

 sclerite " and two adjacent still smaller sclerites. 



We have only to suppose the appendages carrying the gill-books not 

 to have fused as yet in the middle line, and the muscular stigmata to 

 have become greatly developed (perhaps by increased development of 

 the muscle aiding in aquatic respiration) and we have at once the gill- 

 book sinking within the area of the stigmatic pit. 



A very important feature in the supposed further development is 

 the correspondence of the atrophy of the muscle (which atrophy is 

 required to fit in with our hypothesis, and to convert the muscle-pit 

 into a pulmonary sac) with the changes in the structures which would 

 necessarily result were the physiological conditions gradually to 

 become such as to favour aerial in place of aquatic respiration. The 



