Affinities Of Limulus with Living Forms

Limulus (Dorsal & Ventral)

 Phylum :     Arthropoda 
Sub-Phylum:Chelicerata
Class:    Merostomata

Sub-class:Xerophosura
Genus:  Limulus Sp
 

Affinities with Arachnids

Lankester and his associates compared Limulus with Scorpion with many details and established a close relation between them.

SIMILARITIES:

01.               Cephalothorax with 6 pairs of appendages.

02.              1^st pair of abdominal appendages with genital operculum.

03.              Coaxial gland present.

04.              Antennae absent.

05.              Both Lateral and median eyes are present.

DIS-SIMILARITIES

 

01. Book gills are present in Limulus. Book lungs are present in Scorpion/Arachnids.

02. Tail of scorpion bears Poisonous sting but Limulus do not bears stings in their tail.

 

AFFINITIES WITH TRILOBITA

Lankester put Limulus and Trilobita under same phyla .The larvae of Limulus and Fossils of trilobite shows superficial resemblance.

SIMILARITIES:

01. Trilobation of body by pair of longitudinal groove into median lobe & 2 lateral lobe.

02. Lateral eyes present.

03. Presence of Chelicerae, Pedipalpi & chilaria.

04. Abdomen distinctly segmented.

REMARKS:

Limulus differs from trilobite, they do not have antennae and the appendages shows mark difference. While Trilobita have antennae and appendages are same and biramous.

AFFINITIES WITH EURYPTERIDS

01.      Aquatic life in both.

02.      Gills for respiration.

03.      Carapase with Simple median and lateral compound eyes.
04.  Position of appendages are same.

AFFINITIES WITH CRUSTACEANS

Similarities

01.Aquatic life in both.

02.Gills present.

03.Biramous appendages are found in both.
04. Compound eyes present. 

Dis-Similarities
01.Prosoma with 6-7 pair of appendages in limulus, while crustacean have 5 pair of appendages.
02.Limulus with book gills, crustacean with book lungs
03. Antennae present in crustacean, while it is absent in Limulus .  

Conclusion.

From the above mentioned discussion it is quite clear  that  limulus is a close relative to class arachnids and with eurypterids and chelicerates they show distinct and superficial resemblance. The Book gills in limulus is a fore runner of book lungs in scorpion. Also unlike book gills we find that book lungs are internalized into specialized  chambers.

This modification is very significant in translation from aquatic life to terrestrial life. Further modification gave rise to air sacs and trachea in other arthropods groups.

 So it seems proper that limulus like ancestors which were marine gave rise to Scorpions with book lungs  and further evolution gave rise to Air sacs and trachea in terrestrial animals .



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