ZOOLOGY. 351 



MEEOSTOMES OE PAL.EOCAEIDS. 



THE EYES OF THE HORSESHOE CRABS AND TRILOBITES.* 



Professor Packard has investigated the structure of the eyes of the horse- 

 shoe crab, aud, in connection therewith, that of the corresponding organ 

 of the Trilobites. In the eye of tlie horseshoe crab [Limulus) the cor- 

 nea is developed as " a thinned portion of the integument" ; on each side 

 are " nutrient" or pore canals, which are filled with connective tissue, 

 extending into the integument from the body cavity. Apposed to the 

 transparent cornea, and arranged in quincuncial order, are series of solid 

 conical lenses, of which the conical ends project inwards and are partly 

 buried in the black retina. "The long slender optic nerve, just before 

 reaching the eye, subdivides, sending a branch to each facet or cornea 

 impinging on the lens." As this is a unique condition of the eyes in 

 living articulates, it was an interesting i^roblem to discover whether the 

 affinities of the Trilobites with LimnluH was manifested in this as in other 

 respects. Casts and natural sections of Asaphns gigas especially were 

 examined, and when the concave or interior surface was i)laced under a 

 magnifying power of oO diameters, the entire surface was seen to be 

 rough, with the ends of the minute solid conic corneal lenses which pro- 

 jected into the body cavity. "This is exactly comparable with the cast 

 shell of Limulus and its solid corneal lenses which project into the body 



cavity." 



CEUSTACEAXS. 



ABORTED DEVELOP^IENT IN CRUSTACEANS. 



It has long been recognized that while forms inhabiting the sea pass 

 through several stages of metamorphoses, species quite closely related, 

 living in fresh water, do not exhibit similar stages in their development. 

 The forms that so deviate from the general manifestations in the class 

 are always of interest, and Dr. Fritz Muller has examined from this 

 point of view several Brazilian decapod crustaceans.t He found that 

 TrichodacUjlus and Aeglea Odehrechtii failed to pass through the regular 

 crustacean metamorphosis, and that the female of Palcemon potiuna pro- 

 duces only 6 to 20 large eggs, and from these the young emerge with 

 the superficial characteristics of the adults, whose complete form they 

 assume at the fourth change of skin. But what is still more noteworthy 

 is that ^hWelltpjwlyte polar is passes through a shortened phase of devel- 

 opment, on the other hand a Brazilian Hippohjte^ closely related to that 

 species, was ascertained to emerge from the egg as a zooea. Several 

 fresh-water shrimps also found at the mouth of the Itajahy Eiver — a 

 Leander, a rakcmon, and one of the Atyinw — also come from the Q^g as 

 zooeie. 



* Packard, jr. (A. S.). Tlie Structure of the Eye of Trilobites. Am. Saf., vol. cxiv, 

 pp. .50:5-508, July, 1880. 



t Muller (Fritz). Palaemon potiuna ; ein Beispiel abgekiirtzer Verliaudlung. i'ool. 

 Am., vol. iii, pp. 152-157 (223). 



