«t * -f -/. .-(f V '^ 



A wood tick, shown here with its beak inserted into 

 human skin, can transmit serious diseases. This one 

 became attached in a Panama rain forest. Related 

 kinds are common in temperate woods. (Panama. 

 Ralph Buchsbaum ) 



The eight legs of a sea spider ( pycnogonid ) seem 

 joined to one another rather than to a body, for the 

 latter is so very small. The legs accommodate lobes 

 from the digestive tract, perhaps because there is no 

 room for them in the body. (France. Ralph Buchs- 

 baum) 



rate segment of the body. But the skin diver will 

 scarcely be able to detect the animal's unsegmented 

 abdomen, and may wonder whether it has been lost. 

 Actually it is extremely minute, so small that it can 

 provide space for almost no part of the digestive tract. 

 As though in compensation, the legs are somewhat 

 broader toward the base and accommodate lobes of 

 the alimentary canal. 



If the sea spider is a female, she will have a slen- 

 der head ending in a minute sucking mouth — al- 

 most a beak. If a male, he will possess additional ap- 

 pendages on the head, including a pair of small 

 jointed legs held downward and backward. These 

 little legs are the "ovigerous" pair, used by the male 

 to carry ball-shaped masses of eggs laid by his mate. 



At the junction of the head and the first segment 

 of the body bearing walking legs, a sea spider has a 

 small eminence with two to four simple eyes — a 

 turret of visual organs that may keep the animal in- 

 formed of movements of fish and seals in the sur- 

 rounding water. To none of these does the pycno- 

 gonid react in an obvious way. It merely clings, sway- 



ing gently with water movements, or progresses at a 

 sloth's pace from one point to another. 



In coastal waters, sea spiders are mostly small, 

 seldom spanning more than I inch in diameter. The 

 abysses are home to far larger kinds, including the 

 giant Colossendeis, whose legs span as much as 24 

 inches. Pycnogonids have been collected from as 

 deep as twelve thousand feet below the surface, and 

 the five hundred different kinds known include rep- 

 resentatives from all of the world's oceans. They 

 seem particularly common in the frigid waters of the 

 Arctic and Antarctic. 



Sea spiders seem to subsist as adults on micro- 

 scopic algae, but a number of kinds go through juve- 

 nile stages parasitic upon or within coral animals, 

 jellyfishes, nudibranch moliusks, clams, and sea cu- 

 cumbers. The young hatch from the egg equipped 

 with only three pairs of legs. Additional body seg- 

 ments and leg pairs are acquired at metamorphosis. 

 Some pycnogonids do not stop when four pairs have 

 been acquired but continue with the same body plan 

 until they have five or even six pairs of legs. 



[253 



