<p><i>Heterodrilus</i> is a marine group of small (3-25 mm) clitellates that occur interstitially in sandy sediments from the intertidal zone down to about 150 m depths. The taxon includes 42 valid species and has been recorded from localities in the Mediterranean Sea, the North-west Atlantic Ocean (including the Caribbean), the Galapagos Islands, and the Indo-Pacific Region. A majority of the species of <i>Heterodrilus</i> are characterized by having trifid anterior chaetae (i.e., chaetae with three teeth at the distal end). A few species have bifid chaetae, but these are regarded as having lost the third tooth secondarily. Within the taxon, species are distinguished by morphological characters in the internal organization of the male and female genitalia, but also by characters in the form and number of chaetae. In this thesis, two new taxa are introduced, <i>Heterodrilus tripartitus </i>and <i>H. ursulae</i>, both from the Mediterranean Sea. The ultrastructure of the cuticle in four species (<i>H. paucifascis, H. pentcheffi, H</i>. <i>flexuosus, H. minisetosus</i>) is studied and it is shown that there is interspecific variation in the morphology of the cuticle. One of the studied species, <i>H. paucifascis</i>, shows intraspecific variation, which is associated with sample locality. The systematic position of <i>Heterodrilus</i> within Tubificidae is studied with molecular markers, and the results support that <i>Heterodrilus </i>(earlier classified as a member of Rhyacodrilinae) is positioned within Phallodrilinae. Furthermore, the phylogenetic relationships within <i>Heterodrilus</i> are estimated based on molecular characters from mitochondrial COI, the 16S rRNA gene, and the nuclear 18S rRNA gene, and the result indicate that the two major clades in our tree corresponds to different geographical distributions. This thesis also includes a checklist, as well as a key, to the species of <i>Heterodrilus</i>.</p>

Heterodrilus is a marine group of small (3-25 mm) clitellates that occur interstitially in sandy sediments from the intertidal zone down to about 150 m depths. The taxon includes 42 valid species and has been recorded from localities in the Mediterranean Sea, the North-west Atlantic Ocean (including the Caribbean), the Galapagos Islands, and the Indo-Pacific Region. A majority of the species of Heterodrilus are characterized by having trifid anterior chaetae (i.e., chaetae with three teeth at the distal end). A few species have bifid chaetae, but these are regarded as having lost the third tooth secondarily. Within the taxon, species are distinguished by morphological characters in the internal organization of the male and female genitalia, but also by characters in the form and number of chaetae. In this thesis, two new taxa are introduced, Heterodrilus tripartitus and H. ursulae, both from the Mediterranean Sea. The ultrastructure of the cuticle in four species (H. paucifascis, H. pentcheffi, H. flexuosus, H. minisetosus) is studied and it is shown that there is interspecific variation in the morphology of the cuticle. One of the studied species, H. paucifascis, shows intraspecific variation, which is associated with sample locality. The systematic position of Heterodrilus within Tubificidae is studied with molecular markers, and the results support that Heterodrilus (earlier classified as a member of Rhyacodrilinae) is positioned within Phallodrilinae. Furthermore, the phylogenetic relationships within Heterodrilus are estimated based on molecular characters from mitochondrial COI, the 16S rRNA gene, and the nuclear 18S rRNA gene, and the result indicate that the two major clades in our tree corresponds to different geographical distributions. This thesis also includes a checklist, as well as a key, to the species of Heterodrilus.