MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 327 



middle line and are then swept backwards and outwards, describing at each stroke a circle equal 

 in diameter to about twice the length of the body. By the vigorous use of these oars the larva 

 skims rapidly through the water, and its movements are not unlike those of a Gerris upon the 

 surface of a fresh-water pond. 



Notwithstanding the importance of a complete knowledge of the life history of the animal to 

 which this sergestid-like larva belongs, I was unable to complete the study at Beaufort, although 

 I made careful drawings of two stages and filed them away for future use. 



Immediately upon our arrival at Green Turtle Key, in the Bahama Islands, early in June, 

 1886, our attention was at once attracted to a small, graceful, brilliantly colored prawn which was 

 found in abundance among the coral. (See PI. v.) It proved to be Stenopm hispiilus, a species 

 which is chiefly known to naturalists through specimens from the Indian and South Pacific oceans. 

 .It has been recorded as occurring in the tropical Atlantic, but our knowledge of the adult has been 

 very scanty and imperfect, and nothing whatever has been known regarding its life history until 

 Mr. Herrick devoted himself to its thorough investigation. 



It is an active, timid animal, and is one of the most brilliantly colored of the Crustacea. As 

 it is also one of the most widely distributed, it is noteworthy that while its color markings are so 

 prominent and conspicuous they are extremely well fixed and constant; so much so that the speci- 

 mens from the Indian Ocean and the South Pacific agree with those from the West Indies dowu 

 to the most minute markings. 



The adults are found in pairs, a male and a female swimming together side by side and exhib- 

 iting evidence of strong conjugal attachment to each other. 



The most noteworthy fact in its history is its world- wide distribution, and the question whether 

 this can be a result of any peculiarity in its structure or habits at once suggests itself. 



We should expect, on general principles, to find the least specialized species the most widely 

 diffused; and one which holds its ground in so many parts of the world, and without any change 

 of structure finds a safe and congenial home in seas so widely separated, might be expected to be 

 of indefinite or slightly specialized habits, but this is not the case. In structure, in habits, in color, 

 and in external appearance, and also in its metamorphosis, Stenopus is one of the most highly 

 specialized of che crustacea; and it owes its ability to survive in many seas to the accuracy and 

 delicacy of its adjustment to a narrow range of conditions, rather than to indefinite and vague 

 adaptation to many conditions. 



Its antenna? are unusually long and slender, and the acuteness of its senses, together with its 

 very remarkable alertness; the quickness with which it perceives danger, and the rapidity with 

 which it escapes; have undoubtedly aided it in holding its own wherever it has gained a foothold 

 in a suitable locality, and no crustacean, with the exception, possibly, of Gouodactylus chiagra, is 

 better adapted for life in a coral reef. 



It is well protected from enemies by a thorny armor of hooked spines, which cover all the 

 upper surface of its body and limbs, and as all the hooks point forward the attempt of an enemy 

 to swallow a Steuopus must be difficult and painful. 



These facts no doubt account for its survival, and the length of its pelagic larval life is beyond 

 question an aid to its wide dispersal and to the discovery of new homes. 



While we cannot state that the adult will not at sometime be found upon the Atlantic coast of 

 our Southern States, there is no evidence that this is the case, and the larva? which were obtained 

 at Beaufort, North Carolina, were undoubtedly hatched from eggs which were carried upon the 

 abdominal appendages of parents in the West Indies or on the Florida Keys ; and these larva' 

 had therefore wandered more than six hundred miles from their birthplace. The, species might 

 therefore be diffused through a chain of coral islands six hundred miles apart, from a single start- 

 ing point, in a very small number of generations. 



The eggs, which are very small, are laid at night, and the segmentation, which Professor 

 Herrick has thoroughly studied by sections, is entirely confined to the nuclei, the yolk remaining 

 undivided; Steuopus therefore presents a most pronounced type of centrolycethic segmentation. 



The great mass of the egg consists of a homogeneous mass of yolk grannies, which takes no 

 part in the process of segmentation and probably contains no protoplasm. This yolk is aggre- 

 gated around a central nucleus, which divides, probably indirectly, into two, four, eight, sixteen 



