ZOOLOGY. 351 



MEROSTOMES OU PAL^EOCARIDS. 



THE EYES OF THE HORSESHOE CRABS AND TRILOBITES.* 



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

 shoe crab, and, in connection therewith, that of the corresponding- organ 

 of the Trilobites. In the eye of the horseshoe crab {LimnJus) 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 liroblem to discover whether the 

 aflinities of the Trilobites with Limiduii was manifested in this as in other 

 respects. Casts and natural sections of Asa^ilnis (jigas especially were 

 examined, and when the concave or interior surface was placed 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 Hmulns and its solid corneal lenses which project into the body 



cavity." 



CRUSTACEANS. 



ABORTED DEVELOPMENT IN CRUSTACEANS. 



It has long been recognized that while forms inhal>iting 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 Midler lias examined from this 

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

 Trichodactylns and Aeglea Odehrechtii failed to pass through the regular 

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

 duces only C 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 v^hWellippolgfcpoIaris passes through a shortened phase of devel- 

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

 species, was ascertained to emerge from the e^g as a zocea. Several 

 fresh-water shrimi)s also found at the mouth of the Itnjahy River — a 

 Leander, a Falccmon, and one of the Atyimc — also come from the egg as 

 zooese. 



* Packard, jr. (A. S.)- The Structure of the Eye of Trilobites. Am. Xat., vol. cxiv, 

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



tMiiller (Fritz). Pala^mon potiuna ; ein Beispiel abgekiirtzer Verhaudlung. /lool. 

 Anz., vol. iii, pj). 152-157 (223). 



